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Distortion reduction in analogue magnetic recording
1593
Journal of Magnetism and Magnetic Materials 54-57 (1986) 1593-1594 DISTORTION
REDUCTION
IN ANALOGUE
MAGNETIC
RECORDING
R.C. W O O D S Department of Electronic & Electrical Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK
Analogue magnetic recording equipment in common use typically produces : 3% harmonic distortion at peak level. Pulse-width modulation is ideally suited to systems where harmonic distortion must be minimised, and also gives several other incidental advantages. Results show a typical improvement from 1.5 to 0.46% 3rd harmonic distortion at + 10 VU. 1. Introduction
2. Experimental details
Conventional analogue recording equipment generally uses high-frequency ac bias [1] in order to reduce distortion caused by the non-linearity in the magnetisation transfer function. This linearisation is not perfect as currently available analogue recorders produce harmonic distortion reaching = 3% at peak level [2], considerably greater than would be expected from a high quality amplifier (~< 0.1% THD). The use of digital (PCM) systems solves the technical problem but with considerably greater expense both due to additional hardware requirements and also the much greater bandwidth necessary for recording PCM. FM systems offer the potential of low distortion but again at the price of high bandwidth requirements. A system for producing recordings replayable on unmodified conventional analogue equipment but capable of higher quality than obtainable at present would have advantages, as large numbers of tape machines are used primarily for replay only. Pulse-width modulation (PWM) [3] is ideally suited to distortion reduction as even if the transmission chain is non-linear, a linear system is obtained provided that the pulse rise and fall transitions are accurately recorded (giving a bandwidth constraint). This paper examines some consequences of recording using PWM [4] by sampling the input signal at high frequency, generating constant-height current pulses of mark : space ratio proportional to the sampled values of the input signal, and feeding this current drive directly to the recording head [5]. No ac bias is used in the conventional sense. PWM is demodulated simply using a low-pass filter (with a corner frequency between the signal and sampling frequencies), and would be performed automatically by any conventional reproducing equipment. This gives the possibility of making high-quality analogue recordings which are nevertheless replayable using unmodified replay apparatus. This pure PWM technique should offer the twin advantages of inherently lower distortion and no critical bias level adjustment over conventional analogue systems. There may also be an improvement in the signal-to-noise ratio achieved. The bandwidth requirements are considerably below those of FM and PCM systems.
In the experiments reported here, PWM was produced in the usual manner by feeding the input signal, and a linear triangle wave (at the sampling frequency), to the two inputs of a comparator (see fig. 1). The comparator output was then used to drive opposite polarity current sources driving the record head. The head driver functions as a push-pull bridge but its operation is most easily understood by assuming that the right-hand terminal of the head is connected to ground (0 V) rather than to the right-hand side of the bridge. The common-base transistors ( T R I & 2 ) act as opposite polarity current sources, only one of which supplies current at any time. The polarity of the head current is determined by the push-pull pair T R 3 & 4 driven from the PWM signal direct, and the supply voltages shown give a head current of ±12.4 mA, The waveform has a m a r k : s p a c e ratio of unity with no audio input. The maximum current slew-rate through the head is governed by its inductance (0.4 mH in this case) and the maximum voltage available to drive it" because of the inductance, well-formed current pulses are difficult to achieve. By use of the second push-pull pair (TR5&6)
~
udio in
v
+12V
-,2v
+25V
I
-~v
Fig. I. Simplified circuit diagram of PWM recording system. (In practice, protection against overvo]tage, given by diodes not shown, is also needed.)
0 3 0 4 - 8 8 5 3 / 8 6 / $ 0 3 . 5 0 © Elsevier Science P u b l i s h e r s B.V.
1594
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Fig. 2. Frequency spectra for replayed 500 Hz sinewaves, replay level 6 VU [5]. Note the lower spurious signal levels in (al.
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0 2 4 6 8 Replay level / VU
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Fig. 3. Graph of 3rd harmonic level vs. replay' level for PWM and ac biassed recording of a 1 kHz sine wave [5].
tape lnachine's own replay amplifier, introduced negligible distortion (<< 0.1%) so that distortion cancellation c a n n o t be occurring. 4. D i s c u s s i o n and c o n c l u s i o n s
the m a x i n m m drive, and hence the current slew rate, is more than tripled. A transition between current directions is achievable in less than 1 ~s using this circuit. In order to evaluate playback under realistic conditions, a tape machine (Sony TC440) was used with unmodified playback circuitry. For convenience the same machine was used for recording (modified to allow either conventional ac bias techniques or P W M recording). The same record head was used in both cases. A Spectral Dynamics SD335 spectrum analyser was used for h a r m o n i c analysis of waveforms, and a Marconi TF2000 oscillator used as sinewave source ( T H D < 0.05g stated by manufacturer).
Distortion reduction as a result of using P W M has been d e m o n s t r a t e d using a relatively low sampling rate. Since this reduction was observed using unmodified playback equipment it is clear that the technique is especially suited to the production of pre-recorded material, although the sampling rate must obviously be increased if commercial viability is to be attained. Fig. 3 shows that at low replay levels the conventional technique is better than the P W M system described here, and this is attributable to distortion cancellation effects at the corresponding low distortion levels.
3. Results
The author is grateful to Mr. A.J. Harding for help with the measurements, and to Dr. H.K. Kohler for the loan of equipment.
For d e m o n s t r a t i o n purposes, measurements were made using a P W M sampling rate of 17 kHz. Fig. 2 shows typical spectrograms obtained both from conventional ac biassed recording and from P W M recording. The i m p r o v e m e n t in spurious levels using P W M is shown clearly in fig. 3: the 3rd h a r m o n i c distortion level improves from 1.5 to 0.46% for + 1 0 VU at 1 kHz. Measurement showed that the circuit in fig. 1, and the
[1] K.R. Sturley, Sound and Television Broadcasting General Principles (lliffe, London, 1961)p. 134 140. [2] J.C. Mallinson, Proc. IEEE 64 (1976) 196. [3] A.B. Carlson, Communication Systems, 2nd ed.. (McGrawHill, New York, 1981) p. 310-312. [4] T. Fujiwara and K. Yam,mort, IEEE Trans. Magn. MAG-18 (1982) 1244. [5] A.J. Harding, B. Eng. Thesis, Sheffield University, United Kingdom (19841.
Journal of Magnetism and Magnetic Materials 54-57 (1986) 1593-1594 DISTORTION
REDUCTION
IN ANALOGUE
MAGNETIC
RECORDING
R.C. W O O D S Department of Electronic & Electrical Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK
Analogue magnetic recording equipment in common use typically produces : 3% harmonic distortion at peak level. Pulse-width modulation is ideally suited to systems where harmonic distortion must be minimised, and also gives several other incidental advantages. Results show a typical improvement from 1.5 to 0.46% 3rd harmonic distortion at + 10 VU. 1. Introduction
2. Experimental details
Conventional analogue recording equipment generally uses high-frequency ac bias [1] in order to reduce distortion caused by the non-linearity in the magnetisation transfer function. This linearisation is not perfect as currently available analogue recorders produce harmonic distortion reaching = 3% at peak level [2], considerably greater than would be expected from a high quality amplifier (~< 0.1% THD). The use of digital (PCM) systems solves the technical problem but with considerably greater expense both due to additional hardware requirements and also the much greater bandwidth necessary for recording PCM. FM systems offer the potential of low distortion but again at the price of high bandwidth requirements. A system for producing recordings replayable on unmodified conventional analogue equipment but capable of higher quality than obtainable at present would have advantages, as large numbers of tape machines are used primarily for replay only. Pulse-width modulation (PWM) [3] is ideally suited to distortion reduction as even if the transmission chain is non-linear, a linear system is obtained provided that the pulse rise and fall transitions are accurately recorded (giving a bandwidth constraint). This paper examines some consequences of recording using PWM [4] by sampling the input signal at high frequency, generating constant-height current pulses of mark : space ratio proportional to the sampled values of the input signal, and feeding this current drive directly to the recording head [5]. No ac bias is used in the conventional sense. PWM is demodulated simply using a low-pass filter (with a corner frequency between the signal and sampling frequencies), and would be performed automatically by any conventional reproducing equipment. This gives the possibility of making high-quality analogue recordings which are nevertheless replayable using unmodified replay apparatus. This pure PWM technique should offer the twin advantages of inherently lower distortion and no critical bias level adjustment over conventional analogue systems. There may also be an improvement in the signal-to-noise ratio achieved. The bandwidth requirements are considerably below those of FM and PCM systems.
In the experiments reported here, PWM was produced in the usual manner by feeding the input signal, and a linear triangle wave (at the sampling frequency), to the two inputs of a comparator (see fig. 1). The comparator output was then used to drive opposite polarity current sources driving the record head. The head driver functions as a push-pull bridge but its operation is most easily understood by assuming that the right-hand terminal of the head is connected to ground (0 V) rather than to the right-hand side of the bridge. The common-base transistors ( T R I & 2 ) act as opposite polarity current sources, only one of which supplies current at any time. The polarity of the head current is determined by the push-pull pair T R 3 & 4 driven from the PWM signal direct, and the supply voltages shown give a head current of ±12.4 mA, The waveform has a m a r k : s p a c e ratio of unity with no audio input. The maximum current slew-rate through the head is governed by its inductance (0.4 mH in this case) and the maximum voltage available to drive it" because of the inductance, well-formed current pulses are difficult to achieve. By use of the second push-pull pair (TR5&6)
~
udio in
v
+12V
-,2v
+25V
I
-~v
Fig. I. Simplified circuit diagram of PWM recording system. (In practice, protection against overvo]tage, given by diodes not shown, is also needed.)
0 3 0 4 - 8 8 5 3 / 8 6 / $ 0 3 . 5 0 © Elsevier Science P u b l i s h e r s B.V.
1594
R. ('. tlS)octv / Di.s'torlion reduction m a#talogue recording
O-
¢-
3O
--
I
I
I
I
i
1
I
J
E
-20-
ac
e-
2 •g 4o
(a)PWM -40-
/=
/" /-
m
Level/dB
b i a s . "/ /
iN
-600-
./
-~ 50
/
cO
<7:-._.... /
E
-20(b)a.c. bias
,/'PWM
e-
-60
-40-
e.
Level/dB -60I 0
I I I I 500 1000 1500 2000 Frequency / Hz
Fig. 2. Frequency spectra for replayed 500 Hz sinewaves, replay level 6 VU [5]. Note the lower spurious signal levels in (al.
I
d
20 8 - 4
l__l
I
i
_
0 2 4 6 8 Replay level / VU
+10
Fig. 3. Graph of 3rd harmonic level vs. replay' level for PWM and ac biassed recording of a 1 kHz sine wave [5].
tape lnachine's own replay amplifier, introduced negligible distortion (<< 0.1%) so that distortion cancellation c a n n o t be occurring. 4. D i s c u s s i o n and c o n c l u s i o n s
the m a x i n m m drive, and hence the current slew rate, is more than tripled. A transition between current directions is achievable in less than 1 ~s using this circuit. In order to evaluate playback under realistic conditions, a tape machine (Sony TC440) was used with unmodified playback circuitry. For convenience the same machine was used for recording (modified to allow either conventional ac bias techniques or P W M recording). The same record head was used in both cases. A Spectral Dynamics SD335 spectrum analyser was used for h a r m o n i c analysis of waveforms, and a Marconi TF2000 oscillator used as sinewave source ( T H D < 0.05g stated by manufacturer).
Distortion reduction as a result of using P W M has been d e m o n s t r a t e d using a relatively low sampling rate. Since this reduction was observed using unmodified playback equipment it is clear that the technique is especially suited to the production of pre-recorded material, although the sampling rate must obviously be increased if commercial viability is to be attained. Fig. 3 shows that at low replay levels the conventional technique is better than the P W M system described here, and this is attributable to distortion cancellation effects at the corresponding low distortion levels.
3. Results
The author is grateful to Mr. A.J. Harding for help with the measurements, and to Dr. H.K. Kohler for the loan of equipment.
For d e m o n s t r a t i o n purposes, measurements were made using a P W M sampling rate of 17 kHz. Fig. 2 shows typical spectrograms obtained both from conventional ac biassed recording and from P W M recording. The i m p r o v e m e n t in spurious levels using P W M is shown clearly in fig. 3: the 3rd h a r m o n i c distortion level improves from 1.5 to 0.46% for + 1 0 VU at 1 kHz. Measurement showed that the circuit in fig. 1, and the
[1] K.R. Sturley, Sound and Television Broadcasting General Principles (lliffe, London, 1961)p. 134 140. [2] J.C. Mallinson, Proc. IEEE 64 (1976) 196. [3] A.B. Carlson, Communication Systems, 2nd ed.. (McGrawHill, New York, 1981) p. 310-312. [4] T. Fujiwara and K. Yam,mort, IEEE Trans. Magn. MAG-18 (1982) 1244. [5] A.J. Harding, B. Eng. Thesis, Sheffield University, United Kingdom (19841.