admin@hydra.UUCP (admin) (03/10/85)
Can anybody explain to me how the Beta HiFi system avoids
corrupting the stereo hi fi sound signals when it switches from
one spinning video head to the other (which should produce some sort
of phase and amplitude transient every 60th of a second in the FM sound
subcarriers). ?
This transient is invisible in the video because the switch
between heads is in the vertical blanking interval where the machine
generates sync anyway. Audio, on the other hand, is continuous
and the splice cannot be so easily hidden.
I suppose the machine might be so designed that the heads contact
the tape for more than 180 degrees of rotation of the drum, ensuring that
during the splice interval when a switch is made from one head to the
other that both heads are solidly in contact with the tape and producing
high quality signals. Further I suppose that a separate preamp and
sound carrier limiter and detector might be used for each channel
and each head (4 of them) with switching being done between discriminator
outputs rather than FM - RF from the heads as is done with video.
The problem is, how do you ever get discriminators to track each other
that closely ? By putting them all on a chip ? (Isn't it a matter of
external components tracking too ?)
Another scheme is to have separate limiters and RF preamps
as in the first scheme but supply some sort of phase corrector
that would ensure during the splice overlap period that the phase
(and preferably the absolute timing) of the sound subcarriers coming
from both heads was synchronized before a switch was made between sound
carrier source going into the detector. Such a scheme might be
quite simple if all it had to do was correct phase, correcting absolute
timing would involve some sort of variable delay line, however,
which might be more expensive.
A third scheme might be to use some sort of blanking and
interpolation that would simply eat the transient by substituting
the average of values before and after it. This might be done
easily with pulse counting detectors.
Perhaps I have not estimated the magnitude of the switching
transient correctly, but I hear awfully impressive signal to
noise and distortion figures cited for Beta HiFi.
In any case, short of correcting absolute timing (and aligning
carrier phase) it would seem to me that there would be a very slight
but still measureable phase modulation of audio spectral components
at a 60 hz rate, particularly when playing back tapes recorded
on other machines that might have slightly different head to
head delay.
Can anyone comment ?die@hydra.UUCP (Dave Emery) (03/10/85)
Due to a glitch here at CRDS my posting on head switching
transients was unsigned.
Correct signiture follows
--
David I. Emery Charles River Data Systems 617-626-1102
983 Concord St., Framingham, MA 01701.uucp: decvax!frog!diebrown@nic_vax.UUCP (03/11/85)
> > Can anybody explain to me how the Beta HiFi system avoids > corrupting the stereo hi fi sound signals when it switches from > one spinning video head to the other (which should produce some sort > of phase and amplitude transient every 60th of a second in the FM sound > subcarriers). ? I have a VHS Hi-Fi system, the JVC HRD-725U. I also contain the service manuals. The edition date of the one that I have is dated February, 1984. Unfortunately, they did not give the description on what it done about the head switch-over. But I will quote what is printed: A detailed description on the holding circuit for compensation of FM signal dropout and on the head switching signal generator circuit will be given later. One could infer two things from this statement. The techniques of how they do it are a secret for now. But, it sounds as if during head switchover they repeat a portion of the signal. > This transient is invisible in the video because the switch > between heads is in the vertical blanking interval where the machine > generates sync anyway. Audio, on the other hand, is continuous The above statement is partially true. On all helical scan systems, the video head switch-over is done 6.5 horizontal lines (+/- .5) before the vertical equalization pulses start. Roll down the picture using the vertical hold and you will be able to see it. If your set has a vertical hold. A side note on Hi-Fi systems. If you read the consumer video magazines and have seen the add from Sony, you have noticed that they say that their system is better than the VHS system. I won't debate that here. But I will tell about what Sony will not tell the consumer. As you know, or will know, Sony puts their FM carriers between the color sub-carrier and the video sub-carrier. It is done using the same heads that the video uses. On the VHS system, since there isn't enough room to add the FM sub-carriers to the video sub-carriers, separate heads are used. In Europe, the Beta Hi-Fi system from Sony was not released as it was in the states. The reason? Since the PAL and SECAM systems are of wider bandwidth, the FM sub-carriers will not fit within the video sub-carriers. Sony's solution? Use the same technique that is used for VHS, i.e., separate FM audio heads. So much for Sony's idea being better. Mr. Video The above statement was not a paid political announcement by the VHS camp.
sgcpal@watdcsu.UUCP (Paul Layman [EE-SiDIC]) (03/12/85)
> > Can anybody explain to me how the Beta HiFi system avoids > corrupting the stereo hi fi sound signals when it switches from > one spinning video head to the other (which should produce some sort > of phase and amplitude transient every 60th of a second in the FM sound > subcarriers). ? > You might try looking at an article by Kono and Kamai et al. entitled "VIDEO RECORDING AND PLAYBACK SYSTEMS -- BETA HI-FI VCR" in the IEEE Transactions on Consumer Electronics, Vol. CE-29, No. 3, August 1983, pp. 141-151. This article gives the most complete description of Beta HI-Fi that I've seen. The most complete spec's as well ( would you believe a 120dB S/N at 10 kHz !). The most simple explanation I can give you from this article is that there are actually 4 FM carriers -- 2 for each channel. The signal for each audio channel is overlapped on 2 of these channels. The two signals are mixed using a matrix and some select signals form the video heads. The result is "a continuous signal with *no* switching noise." (:-) By the way this article describes a system which was modified somewhat when a new chip-set was introduced by Sony, I think. It was described in the November 1984 Transactions on Consumer elctronics, I think. (My copy is missing). If I recall correctly the performance was different, possibly not as good, but I may be wrong. While I'm at it all you VHS guru's might want to have a look at an article entitled "HIFI VCR System" in the August 1984 issue which describes the VHS HI-FI system in detail. I hope you hope you can understand it. It starts with "In applying this system to VHS VCR's, we have encountered some compatibility problems, since there is no vacant frequency bands for sound signals." And goes on to describe how they finally managed to get it to work in both SP and SLP modes. Unlike Beta they use only 2 audio carriers and make no mention of how they avoide overlapp noise. Paul A. Layman [EE - SiDIC, University of Waterloo]
lauren@vortex.UUCP (Lauren Weinstein) (03/13/85)
By the way, Beta HiFi is accomplished in a totally different manner on most European (e.g. PAL) Beta machines. The European system lays down the audio as a separate "layer" on the tape at a different magnetic "depth" than the video signal (I'm simplifying here, obviously). It's really quite interesting. This alternative technique was chosen to avoid certain compatibility problems. I'm not sure what VHS does in this situation. --Lauren--
keithd@cadovax.UUCP (Keith Doyle) (03/16/85)
[.......]
I heard a rumor that the HiFi audio signals are actually recorded
digitally. If this is true, it is easy to see ways where head
switching would not be a problem. However, I don't really know
if these signals are digital or not. Does anyone else? If they're
not, then how do they achieve any better sound than conventional
audio recorders? (or do they?)
Keith Doyle
# {ucbvax,ihnp4,decvax}!trwrb!cadovax!keithd
"PAYing to know what I REALLY think!"sgcpal@watdcsu.UUCP (Paul Layman [EE-SiDIC]) (03/18/85)
> > [.......] > > I heard a rumor that the HiFi audio signals are actually recorded > digitally. If this is true, it is easy to see ways where head > switching would not be a problem. However, I don't really know > if these signals are digital or not. Does anyone else? If they're > not, then how do they achieve any better sound than conventional > audio recorders? (or do they?) > > Keith Doyle > # {ucbvax,ihnp4,decvax}!trwrb!cadovax!keithd > "PAYing to know what I REALLY think!" They are *not* recorded digitally. They are analog recordings of frequency modulated audio. Wow and flutter is eliminated by the high tape speed, and the signal to noise ratio is improved by using a 2:1 dynamic range compression. Therefore 80 dB of audio dynamic range requires only 40 dB on the recording medium. You could buy a dBX compressor/expander and get roughly the same results on your conventional tape recorder. However you can buy digitizers for use with your VCR. Does anyone own one of these units? What are your opinions on their relative merits? Respond via mail and I'll post to the net. Paul Layman (sgcpal@watdcsu)
heneghan@ihu1m.UUCP (Joe Heneghan) (03/18/85)
> I heard a rumor that the HiFi audio signals are actually recorded > digitally. If this is true, it is easy to see ways where head > switching would not be a problem. However, I don't really know > if these signals are digital or not. Does anyone else? If they're > not, then how do they achieve any better sound than conventional > audio recorders? (or do they?) I think that "digital" is a commonly associated with high quality and that analog is considered as something less. There is no reason to record in digital on a VCR when alalog can give you the performance you need. What gives higher quality in sound or even video is wide bandwith. Lightwave has very high bandwidth which can be utilized in a lot of conversations in the telephone sense or great sound reproduction in the audio sense. VCRs encode a video signal which requires a wider bandwidth than what audio does. Since they're using the video track for audio, this give a lot of bandwidth to encode the signal in a very close way to the original.
jeh@ritcv.UUCP (Jim Heliotis) (03/19/85)
> I heard a rumor that the HiFi audio signals are actually recorded > digitally. If this is true, it is easy to see ways where head > switching would not be a problem. However, I don't really know > if these signals are digital or not. Does anyone else? If they're > not, then how do they achieve any better sound than conventional > audio recorders? (or do they?) They are not digital. You are probably confusing it with pulse-coded modulation methods which use the entire video tape to record a digital (audio-only) signal. Better sound is acheived because information is written in diagonal swipes across the tape, so the head-to-tape speed (H) is much higher than the tape-through-machine speed (M). / ^ head direction on tape / / / / / / / / a ______________________ > tape direction H = M / (cos a) Jim Heliotis {allegra,seismo}!rochester!ritcv!jeh rocksvax!ritcv!jeh ritcv!jeh@Rochester P.S. I feel this is adequate since signal quality ultimately depends on the number of magnetic particles the head sees per unit of time. Other factors determine how close to this ideal one can come.
lauren@vortex.UUCP (Lauren Weinstein) (03/19/85)
One point: while analog and digital systems can certainly provide the same bandwidth, one area where digital will always win out is in avoiding generation loss and gradual degeneration. A properly designed digital audio system, with decent ECC, will provide exactly the same quality from master to consumer, with little chance of audible degeneration over time. --Lauren--
sgcpal@watdcsu.UUCP (Paul Layman [EE-SiDIC]) (03/19/85)
> > I heard a rumor that the HiFi audio signals are actually recorded > > digitally. If this is true, it is easy to see ways where head > > switching would not be a problem. However, I don't really know > > if these signals are digital or not. Does anyone else? If they're > > not, then how do they achieve any better sound than conventional > > audio recorders? (or do they?) > > I think that "digital" is a commonly associated with high quality > and that analog is considered as something less. There is no > reason to record in digital on a VCR when alalog can give you > the performance you need. I think this is a very presumptuous statement. I am looking forward to recorders which encode both the audio and the video digitally. Digital recordings make possible a wide range of error correction and detection schemes to remove those annoying glitches which are present in all commercial analog recorders. > What gives higher quality in > sound or even video is wide bandwith. Not true! > Lightwave has very high > bandwidth which can be utilized in a lot of conversations in > the telephone sense or great sound reproduction in the audio sense. > VCRs encode a video signal which requires a wider bandwidth > than what audio does. Since they're using the video track for > audio, this give a lot of bandwidth to encode the signal > in a very close way to the original. ah!!! Paul Layman (watdcsu!sgcpal)