lionel@eiffel.DEC (Steve Lionel) (08/12/85)
Jerry Askew's tale of the history of sound on videotape was, on the whole, accurate, but omitted some key points and misrepresented a major advance in the industry. As Jerry said, the early VHS format had a 1mm mono linear audio track. This was later split into two .3mm audio tracks for stereo with a .4mm guard band. As you can imagine, .3mm of tape moving at the slow VHS speeds is not conducive to high fidelity, so Dolby B noise reduction was included with this new subformat. All this did was to make the sound a bit quieter. However, Jerry misrepresented the merging of Hi-Fi sound with consumer video. The revolution in video sound was when engineers at Sony (remember them?) realized that if you could write the sound using the fast-moving video heads that you could get a spectacular advance in audio performance. Sony's Beta format (at least in the NTSC version used in the US and Japan) had enough extra bandwidth in the video signal that they could encode the audio in much the same manner as FM stereo and mix it in with the video signal. This new technique, named Beta Hi-Fi, was forwards and backwards compatible with the original, mono linear Beta audio track. Beta Hi-Fi debuted in the Sony SL-5200 (actually built by Aiwa) in the spring of 1983, to be followed by the SL-2700 in August 1983. Now that Sony had, once again, shown innovation in consumer video, what could JVC (the VHS format's keeper) do to catch up? It was well known that VHS did not have the bandwidth to adopt the same scheme that Beta Hi-Fi used, so they came up with the clever method of writing the audio information using separate heads at different frequencies. What seems miraculous to some, the VHS Hi-Fi VCR can pick up both signals with minimal interference between them, and maintain compatibility with non-Hi-Fi VHS tapes. The first VHS Hi-Fi machine on the market was the RCA VKT-550 in the summer of 1984, over a year after Beta Hi-Fi appeared. Though the Beta and VHS Hi-Fi methods differ dramatically in their method of recording the audio, both have approximately the same performance (Beta Hi-Fi machines seem to do a bit better, but the difference is not usually noticeable). You get flat frequency response from 20Hz to 20kHz, negligible wow and flutter, a signal-to-noise ratio of about 75db or better, dynamic range better than 80db and distortion about .3%. Note that since the S/N ratio is so good (compare to a quality LP at 45%), there is no need for noise reduction. However, to get the desired dynamic range, some "companding" is done. Now, we get to the original question. In Beta, there was never an official stereo linear track, so the compatibility problem was not relevant. Beta Hi-Fi machines played and recorded the linear track just fine. All was not so well in VHS-land, however. While there was no reason why a deck couldn't include both the stereo Hi-Fi circuitry and also have the Dolby-encoded stereo linear tracks, some manufacturers opted to economize by only having mono linear capability in their Hi-Fi machines. Thus, on these machines, you can play VHS linear stereo tapes, but not in stereo and without the Dolby processing. Other makers kept the stereo heads, but didn't let you record in stereo on the linear track. Still others allowed full functionality on the linear track. So, you have to be very careful when shopping for a VHS Hi-Fi VCR if you care about linear track compatibility. Well, that's our history lesson for today. Perhaps in a year or so, we'll see if JVC manages to come up with something approximating SuperBeta. From recent announcements, it doesn't seem that way. Steve Lionel (Imagine drawings of boxes labelled Sony SL-2700, Sony STR-VX550 and Proton 600M, with the pithy phrase "Beta Hi-Fi, REALLY the only way to go" (I haven't yet updated that to "SuperBeta HiFi"...))
pjk@hou2a.UUCP (P.KEMP) (08/13/85)
> From: lionel@eiffel.DEC (Steve Lionel) > > Jerry Askew's tale of the history of sound on videotape was, on the whole, > accurate, but omitted some key points and misrepresented a major > advance in the industry. Sorry Steve, but your article could use some clarifications also. I'm no expert in video myself, but I do read the various video rags. > signal. This new technique, named Beta Hi-Fi, was forwards and backwards > compatible with the original, mono linear Beta audio track. Beta Hi-Fi I not sure what you mean by "forwards and backwards", but if you mean that BETA (AFM) HI-FI is upward and downward compatible with mono BETA machines, I must state that while some current BETA VCRs can be upgraded to HI-FI by use of an optional adapter, this is not the case for pre HI-FI models. Therefore BETA HI-FI is not truly upwards compatible, except by expert modifications for older units (READ-> *expensive*). > using separate heads at different frequencies. What seems miraculous to > some, the VHS Hi-Fi VCR can pick up both signals with minimal interference > between them, and maintain compatibility with non-Hi-Fi VHS tapes. The first Please read up more on VHS AFM recording! VHS AFM uses "depth multiplexing" and head azimuth offsets to minimize interference with the video signal. Also, BETA HI-FI for PAL (and SECAM?) systems uses the VHS technique. > wow and flutter, a signal-to-noise ratio of about 75db or better, dynamic > range better than 80db and distortion about .3%. Note that since the > S/N ratio is so good (compare to a quality LP at 45%), there is no need > for noise reduction. However, to get the desired dynamic range, some > "companding" is done. S/N ratio and dynamic range are closely related items. The raw S/N of VHS AFM playback is something like only 40 dB, a noise reduction system called "HD" is used to increase the S/N to 75 dB - 80 dB. I believe it is a companding form of NR. BETA AFM NR is similar, I think, but the "raw" S/N is greater. [Someone correct ME on this if I am wrong!] > Hi-Fi machines played and recorded the linear track just fine. All was not > so well in VHS-land, however. While there was no reason why a deck couldn't > include both the stereo Hi-Fi circuitry and also have the Dolby-encoded > stereo linear tracks, some manufacturers opted to economize by only > having mono linear capability in their Hi-Fi machines. Thus, on these > machines, you can play VHS linear stereo tapes, but not in stereo and > without the Dolby processing. Other makers kept the stereo heads, but > didn't let you record in stereo on the linear track. Still others allowed > full functionality on the linear track. So, you have to be very careful > when shopping for a VHS Hi-Fi VCR if you care about linear track > compatibility. As far as I know, ALMOST ALL VHS HI-FI VCRs can playback the linear audio track in Dolby stereo. Many however, do not RECORD the linear tracks in (Dolby?) stereo. I myself cannot understand this, how much more expensive is it to include the extra circuitry to record in stereo? My VHS VCR, a QUASAR 5845, records the linear audio tracks in Dolby stereo. > > Well, that's our history lesson for today. Perhaps in a year or so, we'll > see if JVC manages to come up with something approximating SuperBeta. From > recent announcements, it doesn't seem that way. > > Steve Lionel > Sorry again Steve, but JVC just announced a picture enhancing technique for VHS. The new process raises the white-clip level from 160% t0 200% in pre-emphasis of the video signal and should make the picture appear sharper, especially at object edges. JVC has quietly included this enhancement in their HR-D140, HR-D250, and HR-D555 VCRs, and has made the technique available to other VHS manufacturers. JVC also working on another technique which could visibly reduce video noise. [This information had been recently posted to this group by someone else.] By the way: Does anyone know if the supposed compatibility problems between HI-FI VCRs [especially between Hitachi and Matsushita (sp?) models] have been eliminated? -- Paul Kemp ihnp4!hou2a!pjk
brown@nicmad.UUCP (08/15/85)
In article <580@hou2a.UUCP> pjk@hou2a.UUCP (P.KEMP) writes: >> From: lionel@eiffel.DEC (Steve Lionel) >> >> Jerry Askew's tale of the history of sound on videotape was, on the whole, >> accurate, but omitted some key points and misrepresented a major >> advance in the industry. > >Sorry Steve, but your article could use some clarifications also. >I'm no expert in video myself, but I do read the various video rags. Time for a little more clarification. >> signal. This new technique, named Beta Hi-Fi, was forwards and backwards >> compatible with the original, mono linear Beta audio track. Beta Hi-Fi > >I not sure what you mean by "forwards and backwards", but if >you mean that BETA (AFM) HI-FI is upward and downward compatible with >mono BETA machines, I must state that while some current BETA >VCRs can be upgraded to HI-FI by use of an optional adapter, >this is not the case for pre HI-FI models. Therefore >BETA HI-FI is not truly upwards compatible, except by >expert modifications for older units (READ-> *expensive*). Backward compatible because older VHS and Beta machine can play the tapes and still have sound. He was talking about the linear track. If is therefore correct in his statement. >> using separate heads at different frequencies. What seems miraculous to >> some, the VHS Hi-Fi VCR can pick up both signals with minimal interference >> between them, and maintain compatibility with non-Hi-Fi VHS tapes. The first > >Please read up more on VHS AFM recording! > >VHS AFM uses "depth multiplexing" and head azimuth offsets to >minimize interference with the video signal. > >Also, BETA HI-FI for PAL (and SECAM?) systems uses the VHS technique. Not only will I read up on VHS Hi-Fi, I will tell you what the VHS Hi-Fi manual for the JVC HR-D725U Technical manual has to say about it. Page 1-12 #T-8015 February 1984 In the VHS Hi-Fi system, the video and audio heads are independent of each other. First the audio signal is recorded and then the video signal is recorded on top of the audio signal. Because of this method, the frequency band of the video signal is never affected by the audio signal and full VHS compatibility is maintained. This is a definite advantage over other Hi-Fi video systems. Generally, the recorded wavelength is determined by l=V/F. l = lambda/wavelength V = relative tape speed of 5.8 m/sec F = Frequency For a FM video signal of 4.4 MHz, the wave-length on the tape is approximately 1.3um. For the carrier frequency of 3.3 MHz, the wavelength is approximately 1.7um. As illustrated [in the book anyway], the recorded wavelength of the video signal ranges from about 1.3 to 1.7um is recorded to a depth of about 0.3um on the magnetic coating of the tape by the 0.3 u gap of the video head. In contrast to this, the FM audio signal for Hi-Fi recording and reproduction has a carrier frequency of 1.3 MHz for channel 1 and 1.7 MHz for channel 2. Converted into th recorded wavelength, they are about 4um for channel 1 and about 3.4 um for channel 2. [The drawing shows a depth of 4 u using a head gap of 1 u. Total tape depth is 20 u.] When the video and FM audio signals are recorded on the same track of the tape, the audio signal magnetizes the tape far deeper than the video signal, as illustrated, and a portion on the deep layer remains even after the video signal is recorded on the surface layer. This means both signals remain with no interference to each other. >> wow and flutter, a signal-to-noise ratio of about 75db or better, dynamic >> range better than 80db and distortion about .3%. Note that since the >> S/N ratio is so good (compare to a quality LP at 45%), there is no need >> for noise reduction. However, to get the desired dynamic range, some >> "companding" is done. > >S/N ratio and dynamic range are closely related items. > >The raw S/N of VHS AFM playback is something like only >40 dB, a noise reduction system called "HD" is used to >increase the S/N to 75 dB - 80 dB. I believe it is a >companding form of NR. The raw dynamic range of VHS Hi-Fi is about 60db, not 40. >BETA AFM NR is similar, I think, but the "raw" S/N is >greater. [Someone correct ME on this if I am wrong!] According to the VHS manual, Beta FM deviation is 75KHz max while VHS has a max FM deviation of 150KHz. This yields about 6db more in raw dynamic range. >> Hi-Fi machines played and recorded the linear track just fine. All was not >> so well in VHS-land, however. While there was no reason why a deck couldn't >> include both the stereo Hi-Fi circuitry and also have the Dolby-encoded >> stereo linear tracks, some manufacturers opted to economize by only >> having mono linear capability in their Hi-Fi machines. Thus, on these >> machines, you can play VHS linear stereo tapes, but not in stereo and >> without the Dolby processing. Other makers kept the stereo heads, but >> didn't let you record in stereo on the linear track. Still others allowed >> full functionality on the linear track. So, you have to be very careful >> when shopping for a VHS Hi-Fi VCR if you care about linear track >> compatibility. > >As far as I know, ALMOST ALL VHS HI-FI VCRs can playback the linear >audio track in Dolby stereo. Many however, do not RECORD the >linear tracks in (Dolby?) stereo. I myself cannot understand >this, how much more expensive is it to include the extra >circuitry to record in stereo? True, most record linear stereo (with Dolby) if they are Hi-Fi stereo, but be sure to ask before buying. Don't assume. The Panasonic PV-9600 in linear mono, with Hi-Fi stereo. Because of its size (portable), there wasn't room for both stereo systems. There are even some table top units that don't record linear stereo, with Hi-Fi stereo. So, don't encourage the video deck makers by buying one. For a little extra information, here are the specs for HR-D725U: Tape width 12.65 +/-0.01 mm Tape Speed (SP) 33.35 +/-0.5% mm/sec Drum Diameter 62 +/-0.01 mm (Upper drum) Writing speed 5.80 m/sec Video Track Pitch 0.058 mm Total video width 10.60 mm Video effective width 10.07 mm Video track center 6.2 mm measured from bottom edge (reference) Video track width 0.058 mm Control track width 0.75 mm Audio track width 1.0 mm single track Audio track width 0.35 mm ch2 (R) top edge Audio track width 0.35 mm ch1 (L) Audio track reference line 11.65 mm measured from reference line Audio to audio guard width 0.3 mm Video track angle 5 deg 56' 7.4" (Stopped) Video track angle 5 deg 58' 9.9" (Running) Video head gap azimuth angle 6 deg +/-10' Positions of audio and control heads from video 79.244 mm FM audio track width Min 0.02 mm FM audio head gap azimuth angle 30 deg I hope all of this helps a little bit. Now, what we really need is digital video recording, for the home market. It is being worked on, but when will it be in the consumer price range. Talk about making lots of generation dubs. -- Mr. Video {seismo!uwvax!|!decvax|!ihnp4}!nicmad!brown