Note: Descriptions are shown in the official language in which they were submitted.
4 . ~
' BACXGROUND OF TE~E INVENTION -- _
16 ~ .
17 The present invention relates to recording on magnetic
18 media and more particularly to a system for varying the recording
19 bias dynamically so that at each instant the bias is more nearly
optimized for the spectrum of material to be recorded than when
21 a fixed bias is employed. In the magnetic recording of analog
22 signals, for example audio, it is normal to employ high frequency
23 recording bias to linearize the otherwise highly nonlinear medium.
24 The current applied to the recording he~d then consists of the
: 25 addition of a current representing~the analog signal to be recorde
26 and a bias current waveform, normally sinusoidal, at a frequency
27 several times higher than the highest frequency component of the
28 analog signal. The magnitude of the bias current affects various
29 parameters oflth~ recoraing system: signal-to-noise ratio,
harmonic distortion, frequency response, sensitivity, output level
31
32
llZZ70Z
1 ¦ modulation noise and the incidence of drop-out ~momentary drops in
2 ¦ output level due to inconsistencies in the magnetic tape and in
¦ its contact with the recording head). For many typical recording
4 ¦ situations, particularly those where the recording medium is a
5 ¦ Compact Cassette, the primary concern is a conflicting requirement
6 I between a magnitude of bias current that results in minimum
7 ¦ distortion at low and middle audio frequencies along with minimum
8 ¦ drop-outs and a lower bias current magnitude that provides a
9 ¦ higher maximum output leYel at high audio frequencies. Heretofore ,
10 ¦ bias level has been chosen as a compromise among the various
11 ¦ tape parameters that vary with the bias level. In the case of
12 Compact Cassette recording,tape saturation occurs at high audio
13 ¦ frequencies,thus limiting the high frequency maximum output level
14 ¦ and causing an audible loss in the high frequency range of
15 ¦ reproduced tapes. ~Iigh frequency tape saturation in Compact
16 ¦ Cassette recording can be substantially reduced, although apparent _
17 ¦ ly not completely eliminated by optimizing record bias for that
18 result. Although saturation does not generally occur at high
19 ¦ frequencies in reel-to-reel tape recorders operated at profession-
¦ al speeds, for example, 38 cm/sec (l5 ips) or l9 cm (7-1~2 ips),
21 ¦ the high frequency output level and other record parameters of
22 I such recorders are enhanced by bias current levels lower than
23 ¦ compromise values generally used. However, the low and middle
24 ¦ frequency distortion, drop-out rate and other parameters are
degraded by such lower bias magnitude.
26 Since the conflicting requirement between a bias levPl
27 chosen for optimum low and mid-frequency audio versus a bias level
28 chosen for optimum high frequency audio output level is basic
29 to typical relcording situations and because the results of that
31
-2-
. l
-
ll'ZZ70Z
1 selection are so readily audible in the reproduced recording, the
2 emphasis of the application and the preferred embodiments are
3 directed to the resolution of the conflict. Those of ordinary
4 skill in the art will appreciate that circumstances, particu-
larly the type of magnetic medium, effective recording speed or
6 specially desired results may cause other record parameters to
7 be of greater importance.
8 ~hile the selection of a preferred bias level at any
9 given frequency still re~uires a compromise, because the
parameters tend to be optimum at different bias levels even ~or
11 a given frequency, a substantial improvement in overall perform-
12 ance can nevertheless result compared to a compromise bias level
13 for the entire frequency spectrum of interest to be recorded.
14 ~he choice of bias level therefore reguires information on the
frequency spectrum to be recorded.
16 ' It should be noted that for each value of bias current
17 the recording amplifier gain and equalization can be set to give
18 the desired level of recorded magnetic flux and the desired
19 overall (record-playback) frequency response, provided of course
that the input level is not so high as to result in tape non-
21 linearity.
22
:23 SUMMARY OF THE INVENTION
In accordance with the teachings of the present
invention these and other problems in the prior art are overcome
27 by a system for recording on a magnetic medium in which the
28 recording bias is dynamically varied to more nearly optimize
the bias for the spectrum of the material to be recorded.
29 To maintain a desired overall record-playback frequency response,
31
32 -3-
,, , . . ,
. . . . .
" liZZ7()2
the recording amplifier characteristics can be dynamically
varied as the bias is varied.
The invention pertains to apparatus for recording
~udio signals on a magnetic medium including amplifier means
receiving the audio signals, means responsive to the audio
: signals for generating a record bias signal varying in response
to at least one predetermined parameter of the audio signals,
the generating means causing the record bias signal to vary
more slowly than variations in the at least one predetermined
parameter of the signals, and means in recording relationship
with the magnetic medium for applying the amplified audio
signals and said varying record bias signal to said magnetic
medium. :
The invention also relates to a system for recording
audio signals on a magnetic medium having high frequency com-
pressor means receiving the audio signals, the means including
~: means for generating a control signal responsive to the high
frequency components of the audio signals to control the high
frequency compression, amplifier means receiving the high
frequency compressed audio signals from the compressor means,
means further responsive to the control signal for generating a
record bias signal varying in response to the control signal,
and means in recording relationship with the magnetic medium
for applying the amplified audio signals and the varying record
bias signal to the magnetic medium.
The invention further relates to apparatus for record-
ing analog signals on a magnetic medium including amplifier
means receiving the analog signals, means responsive to the
analog signals for generating a record bias signal varying in
response to at least one predetermined parameter of the analog
signals, means responsive to the analog signals for varying the
equalization of said amplifier means in response to at least
~lZ270Z
one predetermined parameter of the analog signals, and means
in recording relationship with the magnetic medium for applying
the amplified audio signals and the varying record bias signal
to the magnetic medium.
More particularly, a quiescent bias level is provided
that is substantially optimized for one frequency spectrum of
the information to be recorded. The amplitude of the inform-
ation to be recorded in a further frequency spectrum is monitor-
ed and when that amplitude exceeds a predetermined value the
bias level is varied from its quiescent level to more nearly
optimize the bias level for the further frequency spectrum.
Additionally, ~he record amplifier gain and equalization can be
simultaneously varied with the bias in order to maintain a more
invarying record/playback response.
According to a preferred embodiment, the invention is
employed in an audio tape recording system in which the quiescent
bias level is chosen to provide substantially the best results
at low and mid-audio frequencies. The high frequency level of
the signal to be recorded is monitored and when it exceeds a
predetermined magnitude, the bias is reduced from its quiescent
value. Since the bias change affects frequency response, the
record amplifier gain and equalization are appropriately altered
as the bias is reduced. An overall improvement in the inform-
ation recorded on the tape is thereby achieved.
These and other advantages of the present invention
will be better appreciated as the following detailed description
is read in conjunction with the drawings.
-4a-
,,
llZZ702
BRIEF ~ESCRIPTION OF THE DRAWINGS
-
Figure 1 is a block diagram showing an analog recording system em-
bodying the invention.
Figure 2 is a block diagram showing the control signal generator of
Figure 1 in greate~ detail.
Figure 3 is a block diagram showing an alternate control signal
generator.
Figure 4 is a set of exemplary curves relating recording bias ampli-
tude to playback output level for high speed audio recording.
Figure 5 is a set of exemplary curves relating recording bias ampli-
tude to playback output ievel for low speed audio recording.
Figure 6 is a block diagram showing one means of independent control
of recording bias in multi-track recorders.
Figure 7 is a partially block schematic diagram showing a circuit
usable for providing variable bias in an audio recorder.
Figure 8 is a schematic diagram showing a circuit usable for provid-
ing variable equalization in an audio recorder.
Figure 9a is a schematic diagram showing a variable attenuator using
a bipolar transistor.
Figure 9b is a schematic diagram showing a variable attenuator using
a field effect transistor.
Figure 10 is a schematic diagram showing a variable attenuator using
a light dependent resistor.
Figure 11 is a partially block schematic diagram showing an alterna-
tive to Figure 7.
Figure 12 is a set of exemplary curves relating playback output
level to frequency response at a fixed distortion level for several choices of
bias amplitude.
llZZ70Z
1 ¦ DETAILED DESCRIPTION OF T~E INVENTION
2 l
3 ¦ Referring now to the drawings and particularly to
4 ¦ Figure l which shows a basic block diagram of the invention
5 1 in the context of an analog recording system for recording signal
6 ¦ in the audio spectrum on a magnetic medium. As in a conventional
7 ¦ recorder, the input signal passes via a recording amplifier 1
8 ¦ having an amplification and frequency response determined by
9 ¦ gain and equalization components ~epresented by block 2 to a
10 ¦ summing means 4 and thence to the recording head 5 in a recording
i1 ¦ relation w$th magnetic medium 7, which is magnetic tape in this
12 ¦ example. A high frequency (roughly an order of magnitude higher
13 ¦ than the highest frequency to be recorded, typically about lOO
14 ¦ kHz for audio recorders) oscillator 3 generates the recording bia
15 ¦ waveform which is also fed to the recording head 5 via summer 4.
16 ¦ However, in the present invention the recording e~ualization
17 ¦ and gain, determined by block Z, and the amplitude of the
18 ¦ recording bias generated by block 3 can be varied by means of a
19 ¦ control signal produced by block 6. The v~ e of this control
20 ¦ signal is determined by the spectrum and amplitude of the analog
21 ¦ input signal in such a way that as the high frequency content of
22 ¦ the input increases above a predetermined threshold, the control
:23 ¦ signal operates to reduce the recording bias level generated by
24 ¦ block 3 from its quiescent value, thereby permitting the recordin
of high levels of high frequencies~on the tape, and also to alter
26 ¦ the gain and equalization set by block 2 so as to maintain an
27 ¦ overall flat record-playback frequency response and a constant
28 recording ~ensitivity. The control signal gonerator 6 may
29 ¦ alternativel~ be ~ed from the output of the recording amplifier l
rather than from its input.
32
-6-
,,
llZZ70Z
1 ¦ Further details of the control signal generator 6 are
2 ¦ shown in Figure 2. The signal to be recorded is fed via a high-
3 ¦ pass filter 10 whose characteristic is derived from the high
4 frequency overload characteristics of the tape, to a rectifier 11
s ¦ The resultant unidirectional signal is smoothed in block 12 and
6 passes to a threshold circuit 13 whi~h only gives an output when
7 ¦ its input exceeds some value corresponding to an original high
¦ frequency lnput signal which would overload the tape with the
9 ¦ recording bias at its quiescent value. As described above, the
10 ¦ output of block 13, the control signal, results in a reduced
11 ¦ recording bias and therefore prevents or reduces the overload,
12 ¦ at the same time altering the recording parameters to prevent
13 changes in overall frequency response and sensitivity.
14 ¦ Low-speed audio tape recorders, using Compact Cassettes
for example, frequently employ a type of noise reduction system
¦ designated commercially as the Dolby B-Type noise reduction systel ,
17 whose compressor, employed during recording, contains a high-pass
18 ¦ filter, rectifier and smoothing means similar to those of Figure .
19 It is therefore possible to derive the control signal for the
20 ¦ variable recording bias gain and equalization from the Dolby
21 ¦ B-Type compessor as indicated in Figure 3. Blocks 15 to 23
22 ¦ inclusive constitute a normal Dolby B-Type compressor. By
;23 comparing Figures 2 and 3 it will be seen that blocks 15 to 17
24 ¦ and 19 to 21 carry out similar operation to blocks 10 to 12, and
that, therefore, a control signal,~to operate on the recording and
26 equalization can be generated by the addition of a threshold-
27 ¦ determining circuit 25 corresponding to block 13 of Figure 2, and
2~ ¦ optionally, a further smoothing means 24. T~e threshold-deter-
29 mining circu~t 24 may alternatively be fed from the output of the
31 ¦ ,
32 l
I -7-
112Z702
1 second smoothing means 23, when the further smoothing means 25
2 ¦ is omitted.
3 ¦ Various control signal circuit configurations will be
4 ¦ apparent to those of ordinary skill in the art depending in part
5 ¦ on the level of sophistication desired and permissible cost. In
6 any case, the generation of a control signal is merely an
7 ¦ intermediate step in causing the record bias and, usually, the
8 1 record gain and equalization to vary in a way that results in a
9 ¦ recorded signal more faithful to ,the applied signal than if a
10 ¦ fixed bias were employed. Some circuits may be less effective
11 1 than others, for reasons of cost, for example, but will still
12 ¦ result in improved recordings.
13 ¦ For optimum results the operation of varying the
14 ¦ recording bias and other characteristics should be performed
15 ~ independently in each channel of a multi-track recorder, so as
16 to prevent the modulation of the recording parameters of one
17 ¦ channel by material being recorded on other tracks. In the
18 1 app}ication to stereo recording, where there is a considerable
19 degree of correlation between the material to be recorded on the
20 ¦ two tracks, it may sometimes be permissible to modify the
21 ¦ recording parameters of both channels together in accordance
22 1 with one control signal derived from some combination of the
:23 ¦ material ln the two channels, for example the sum of the channels
24 or the gxeater of the channels.
25 ¦ The ideal function rela~ng the amplitude of the bias
26 current to,tpe level and spectrum of the signal to be recorded is
27 complex, and will be different for every type and brand of
28 recording tap'e. In tape media in which therè is a value of
29 recording bils which permits linear recording of the highest
31 levels and highest fre~uencies present in ~e aFtual material to
32 1 _3_
Il'ZZ70Z
1 ¦ be recorded, this value however being less than the quiescent
21 value normally employed to improve other factors of the recording
3¦ process, the bias-changing function should be such that the workir g
4 point of the tape is adjusted to be always below saturation, that
¦ is such that the tape remains linear. It will then usually be
6¦ necessary to introduce variable recording gain and equalization
7 ¦ to maintain the frequency response and sensitivity as the bias
8 ¦ amplitude changes. This state of affair applies for example to
9 ¦ recording at 38 and 19 cm/sec (15~and 7-1/2 ip9, respectively)
10 ¦ using the NAB or CCIR recording equalization standards.
11 ¦ In tape media in which there is no value of recording
12 ¦ bias permitting linear operation with high level high frequency
13 ¦ signals, saturation is inevitable, and variable bias cannot
14 ¦ eliminate non-linearity but can only reduce its extent. The
bias-changing function should then be such as to minimize the
16 ¦ degree of tape saturation up to as high an input level as
17 ¦ feasible. In this case, the tape medium remains "imperfect",
18 the imperfections only being reduced somewhat, and the extra
19 ¦ compliexity of variable equalization and gain may be unnecessary.
This state of affairs applies to low-speed recording such as in
21 ¦ the Compact Cassette system.
22 ¦ The quiescent recording bias, and therefore the quies-
:23 ¦ cent recording equalization and amplification, may not be those
24 ¦ which would have been adopted in a recorder employing fixed bias,
since the conventional compromises~between high frequency
26 ¦ saturation and other aspects of the recording performance no
27 ¦ longer need apply. In general variable bias will permit a
28 ¦ higher quiescent bias amplitude, leading to lower distortion at
29 low and midd~e frequencies. This can allow an increase in
31 I
32 I _g_
~ llZZ70Z
1 ¦ maximum recording levels, and result in an improvement in signal-
2 ¦ ~o-noise ratio.
3 ¦ Figure 4 illustrates how the sensitivity and maximum
4 ¦ undisto~ted output level of a recorder running at 38 cm/sec vary
5 ¦ with recording bias amplitude for a typical magnetic tape used at
6 ¦ such a speed in a system employing no~mal NAB or CCIR recording
7 ¦ characteristics. Quiescent bias amplitude 'a' may be employed to
8 ¦permit the recording of high levels at low and middle frequencies.
9 ¦ ~owever, with this value of bias, the maximum output ! at 15 k~z (N)
10 ¦is less than that at lower frequencies (M) and on material contain
i1 ¦ing very large amplitudes at high frequencies (bright material)
12 ¦ distortion may occur. If the bias is reduced to, for example,
13 ¦ amplitude 'b' during such passages, the high frequency maximum
14 ¦output level is increased and distortion is reduced or prevented.
15 ¦In reducing the bias from 'a' to 'b', the sensitivities at 1 k~Iz
16 ¦and 15,k~z increase by about 1 and 3 dB respectively, and as a
17 Iresult it is desirable to reduce the gain and high-frequency boost
.81 in the recording amplifier to maintain the overall sensitivity and
19 frequency unchanged. ~he amount by which the bias should be
20 ¦reduced depends on how much the high frequency components of the
21 ¦material exceed the high frequency maximum output level curves.
22 ¦In such a high speed recorder, only under very stringent program
23 conditions will bias reduction be necessary.
24 ¦ ~owever, in low speed recording bias reduction is
25 ¦required far more frequently. Figure S shows similar information
26 ¦to Figure 4, but for 4.76 cm/sec (e.g. Compact Cassette). Con-
27 ¦ventionally the choice of a recording bias amplitude involves a
28 ¦compromise between maximum output level at lo~,and middle fre-
29 ¦quencies and ~aximum output level at high frequencies. A typical
30 ¦compromise is amplitude 'c', with which the maximum output at
31 10 kHz is 10 dB less than at 333 ~z. An inevitable result is
32 l
Z70Z
high frequency saturation on bright material. By decreasing the bias in
accordance with the high frequency content of the material to be recorded, a
higher level of high frequencies can be accommodated. For example, a reduc-
tion of bias to level 'd' permits the recording of 10 kHz signals 4 dB higher.
This change in bias is accompanied by a loss in low and middle frequency sensi-
tivity of about 0.5 dB and an increase in 10 kHz sensitivity of about 3 dB;
preferably, the gain and equalization in the recording amplitude should be
modified accordingly. Since with no value of bias can the maximum output
level at 10 kHz ever reach the maximum output level at 333 Hz obtained with the
quiescent bias 'c', bright material will still give rise to saturation. The
variable bias can only reduce the duration and amount of saturation.
Measurement of twin-tone intermodulation distortion similarly re-
veals the benefit of varying the recording bias. Figure 12 illustrates the
level which can be recorded on compact cassette tape for a fixed proportion
(3%) of third order intermodulation distortion, as a function of frequency and
with three different values of recording bias. Curve a represents the typical
performance when a fixed compromise value of bias is employed ~as in conven-
tional recorders). When the input signal contains large amplitudes at high
frequencies, the recording bias may be reduced to permit recording with much
lower distortion (see curves b and c).
As explained above, in general a recorder capable of recording on
more than one track at a time will require independent control of the record-
ing bias for each track. The recording bias generator, block 3 in Figure l,
may then take the form shown in Figure 6. A master oscillator 30, for example,
that which generates the high frequency power applied to the erase head to
wipe the tape clean of any previous recording, is fed to a number of voltage-
controlled amplifiers 31 a, b, c, one for each track. A plurality of control
information generators like those shown in Figures 2 and 3, one for each track,
operate on the appropriate voltage-controlled amplifier 31 to produce a vari-
able amplitude signal, which passes to an output amplifier 32, one for each
l~ZZ70Z
track, The output of amplifier 32 ls the variable bias current
applied to the recording head as indicated in Figure l.
The voltage controlled amplifiers 31 may alternatively be
replaced by variable attenuators employing voltage-controlled resis-
tances. If the circuit configuration is arranged so that the recording
bias voltage is small (for example, a few tens of millivolts) at the
point where its amplitude is controlled, bipolar or field-effect tran-
sistors (such as Q4 or Q5, respectively~ may be employed as the vari-
able elements. Figures 9a and 9b show examples~ In other instances,
particularly when the bias voltage to be controlled is large (for
example, several volts), a light-dependent resistor, such as the de-
vices fabricated of cadmium sulfide or cadmium selenide, may be em-
ployed to vary the bias amplitude; the control voltage is then em-
ployed to alter the brightness of a light source 34, either incan-
descent or more probably semiconductor, shining on the light-
dependent resistor. In some embodiments the bias output amplifier
32 can be eliminated, and the recording bias applied directly to
the recording head via a light-dependent resistor 36, as in Figure
10.
In all these cases the bias-varying mechanism can be
employed additionally to turn on and off the bias smoothly at the
begin~ing and end of recording, and to pre-select different values
of quiescent bias current to suit different tape speeds or types.
In embodiments where it is acceptable to control the
bias amplitude of several record tracks together, a much simpler
technique is to vary the power supply to the bias oscillator.
Figure 7 show~ one practical circuit operating in this manner,
_ lla -
llZ270Z
1 ¦ or use in a stereo recorder containing Dolby B-T~pe noise
2¦ reduction. Diodes Dl and D2 ~eed the greater of the outputs of
31 the firs~ smoothing means of the left and right channel Dolby
4¦ compressors to the gate o~ field-effect transistor Ql operating
51 as a source follower and level shifter. The signal from the
6 ¦source of Ql is applied to theinput ok operational amplifier
7 ¦A1 having an inverting gain of approximately 30 times at zero
8 ¦frequency and incorporating further smoothing by means of the
9 ¦parallel combination o4 resistors R4 and capacitor Cl~ in its
10 ¦feedback path; the time-constant o~ these components is about
11 ¦l ms, appreciably shorter than the fastest attack time of a
12 ¦Dolby B-Type compres~or. The non-inverting input of amplifier Al
13 ¦is connected to a potential set by potential divider RVl. The
14 ¦ampli~ier output is connected via diode D3 and resistor RS in
15 ¦qeries to the power supply input connection of the bias oscillator
16 which draws its power through xesistor R6. Under ~uiescent
17 ¦conditions the output potential of the amplifier is sufficiently
18 ¦positive that diode D3 is reverse-biased and the supply to the
19 ¦bias oscillator is determined by the value of resistor R6. As
20 ¦signal levels a~d frequencies at the input of the recorder are
21 ¦ increased, the gate and therefore the source of Ql move positively
22 ¦-and hence the output of amplifier Al moves negatively. The
23 ¦ threshold control RVl is set so that at the approach of tape
Z4 ¦ saturation at high frequencies, diode D3 becomes forward-biased,
25 ¦ allowing amplifier Al to draw currept away from the oscillator
26 ¦ via resistor R5, reducing the supply potential to the oscillator,
27 ¦ and hence reducing the recording bias. As signal levels are
28 ¦ progressively i~creased, the output of Al fal~s more and draws
23 ¦ more current way from the oscillator, progrsssively reduoing
31
I -12-
I
- -
" llZZ702
the recording bias~ Eventually, the output of Al cannot go any further nega-
tive, and a minimum value of bias is reached.
An alternative circuit is shown in Figure 11. As in Figure 7,
diodes Dl and D2 select the greater of the outputs of the first smoothing
means of the left and right channel Dolby compressors. Amplifier A2 amplifies
this signal and shifts its level by a voltage determined by the setting of
thresholds control RV2. When the output of amplifier A2 becomes positive, it
is passed via diode D4 to amplifier A4 which provides a further smoothing
means and additional amplification. The power supply voltage for the bias
oscillator is the difference between the main power supply tin this example,
15 volts) and the potential at the output of amplifier A3. Hence as the
signals from the Dolby compressors move positively, the supply applied to the
bias oscillator is reduced, and so the recording bias amplitude decreases.
When the output potential of amplifier A3 approaches its positive power supply
(in this example, 8 volts), the amplifier saturates, so providing a limit to
the degree of reduction of bias amplitude.
The method of varying recording bias by changing the power applied
to the bias oscillator may also be employed when the bias variation foT each
channel is required to be independent of other channels. In this case separate
circuits, for example of the form shown in Figures 7 or 11, are employed for
each channel. The bias oscillators will normally be lightly coupled together,
so that they operate at the same frequency.
Figure 8 shows one method of varying the recording equalization.
Transistor Q2 is the recording amplifier with frequency-dependent feedback
in its emittter circuit. The damping of the series resonant tuned circuit
provided by R8 C2 Ll is varied hy the resistance of the field-effect transistor
Q3 in such a way that as the recording bias and equalization control voltage
apPlied to the FET gate is made more positive, the damping is progressively
increased, and the quantity of the treble boost applied to the signal passing
to the recording head is thereby decreased.
~ ~ z;Z702
Those of ordinary skill in the art will appreciate that various
modiications within the spirit of the invention can be made to the preferred
embodiments dlsclosed. The invention is therefore to be limited only by the
scope of the appended claims.
