VHS, SVHS modulation techniques
Bob Nixon
I don't know if this is the appropriate place to pose this question, if not
someone please direct me to a better newsgroup or other Internet based
information source.
I remember reading in the early days of Hi-FI VCR's that the HI-FI (FM)
sound tract is DEPTH modulated on the media. I also was under the
impression that VHS & SVHS separate the luminance and chrominance
information. Can someone please post a detailed abstract of the VHS
modulation scheme. TIA
--
Big...@Primenet.com
(Bob Nixon)
Gary L. Sanders
The chroma is separated from the luminance by low-pass filtering the
luminance and bandpass filtering the chroma in most VHS machines. High
end VHS and all S-SHS machines use comb filtering to separate chroma and
luminance (better). So far, machines use single line comb filters, but
the new 3-line adaptive comb filter chips should be showing up, like in
the newer high-end TVs.
The chroma which was originally centered around 3.58MHz with a
bandwidth of 1.2 MHz, and was double-sideband, surpressed carrier
quadrature modulated. On the record side, it is down-converted to 629kHz.
The bandwidth is reduced to around 500kHz, so that there is space for
both upper and lower sidebands at the new lower carrier frequency.
The luminance is FM-modulated and used as the bias signal to put the
chroma down on tape. Pre-emphasis is used on the luminance on recording
and de-emphasis when playing back. After pre-emphasis, large
black-to-white transitions have big overshoots that could overmodulate
the tape and cause an RF dropout when played back. To prevent this,
peak-white clipping is used. Upon playback, however, the missing
overshoot results in a slow transition after de-emphasis is applied.
Heads with smaller gaps and better tape has resulted in less amounts of
peak-white clipping being used, resulting in snappier white titling.
Playback noise-coring is also used to improve the S/N ratio at the
expense of low-level high frequency detail. The luminance high
frequencies are separated and fed through clipping diodes to remove the
low-level high frequencies. This signal is added back to the
low-frequency luminance. Early VHS machines had lots of this, resulting
in cartoony, flat-looking textures. Although less noise-coring is
used today, due to better tape and heads; high-end machines and S-VHS
machines have a compensating circuit in the record side to place
low-level high-frequency video on a pedastel, so that it cannot be lost
by the noise coring circuit on playback. This is the video analogy to
Dolby processing applied to video. For years, the "Dolby" preprocesser
has been missing in VCRs.
Before the luminance and chroma are recorded, the stereo hi-fi tracks are
FM modulated at 1.3 and 1.7 (I think) MHz, they are recorded first, and
deeply into the tape; then the video is recorded over them. This is what
they mean by "depth" modulation, it is not "variable" depth, but recorded
deeper into the tape than the video.
S-VHS has just slightly wider chroma bandwidth and higher FM carrier
frequencies for the luminance so that the wider-bandwidth combed
luminance can be recorded. S-VHS machines also have circuits that speed
up the slow chroma transitions. The bandwidth is not really increased,
but the chroma looks sharper.
The linear track(s) are recorded with bias like regular cassette audio.
Some years ago, machines had Dolby Stereo available on the linear tracks,
but this was eliminated to reduce royalty costs when hi-fi stereo came
out.
--
Gary L. Sanders, V.P. Engineering
Sanders Media Adventures, Inc.
Video Design Consulting
C-MAC/SMA Active, Hybrid Video Filters
http://ourworld.compuserve.com/homepages/gls_sma
phone/fax (408) 279-5118
Ed Ellers
Gary L. Sanders wrote:
> The chroma is separated from the luminance by low-pass filtering the
> luminance and bandpass filtering the chroma in most VHS machines. High
> end VHS and all S-SHS machines use comb filtering to separate chroma and
> luminance (better). So far, machines use single line comb filters, but
> the new 3-line adaptive comb filter chips should be showing up, like in
> the newer high-end TVs.
A lot of consumer S-VHS VCRs have used these 2H adaptive comb filters in
the record path for several years now; one of the first was the JVC
HR-S6600, introduced in 1989. (Unfortunately I haven't heard of any
using an adaptive comb in the *playback* chroma path, which would cure
the downward smearing caused by the use of a comb filter to remove
crosstalk.)
> S-VHS has just slightly wider chroma bandwidth and higher FM carrier
> frequencies for the luminance so that the wider-bandwidth combed
> luminance can be recorded. S-VHS machines also have circuits that speed
> up the slow chroma transitions. The bandwidth is not really increased,
> but the chroma looks sharper.
*Some* consumer S-VHS decks, mostly later JVC models (starting with the
HR-S6700 in 1991), have chroma transition improvement. Most other makes
don't from what I've seen, at least not those sold in North America.
> The linear track(s) are recorded with bias like regular cassette audio.
> Some years ago, machines had Dolby Stereo available on the linear tracks,
> but this was eliminated to reduce royalty costs when hi-fi stereo came
> out.
Most of the first Hi-Fi VHS VCRs had Dolby B NR as well -- including
machines such as the Panasonic PV-1730 and JVC HR-D725 -- but the
feature was dropped in most cases a year later (though JVC's HR-D756
also had Dolby B NR). Incidentally, this feature is not properly
called "Dolby Stereo," since that term refers only to theatrical film
sound and not to any consumer format.
Andrew Mai
In article <322E69...@ix.netcom.com> "Gary L. Sanders"
<gls...@ix.netcom.com> writes:
...[a great article about signal modulation for VHS and S-VHS formats]
>The linear track(s) are recorded with bias like regular cassette audio.
>Some years ago, machines had Dolby Stereo available on the linear tracks,
>but this was eliminated to reduce royalty costs when hi-fi stereo came
>out.
Just a small nit-pick: Dolby Stereo is the old logo for the theater
surround sound which came from the Dolby labs. What the linear tracks
of some pre-recorded tapes used to have is Dolby B NR (noise reduction),
also from Dolby labs.
(No, I do not now nor have I ever worked for Dolby.)
Andy Mai
m...@ncar.ucar.edu
Gary L. Sanders
Well, what I meant was Dolby B for stereo tracks.
Bob Nixon
"Gary L. Sanders" <gls...@ix.netcom.com> wrote:
Many thanks Gary. This is exactly what I was looking for. One
more small question please. When the FM modulated audio is
recorded on the diagonally scanning tape, how is the audio put
back together during playback? It seems like audio unlike video
is serial or continuous whereas video is a natural for this type
of one frames information for each swipe across the tape.
It were digital PCM I could see no problems but it's FM.
Related issue:
On poorly recorded tapes the sound track sometimes has a buzz
(tracking related). Is this a failure to put this audio track
(back together) properly?
TIA
>The chroma is separated from the luminance by low-pass filtering the
>luminance and bandpass filtering the chroma in most VHS machines.
Snipped-------------------------------------------------------------------------->
>Bob Nixon wrote:
>>
>> I don't know if this is the appropriate place to pose this question,
>Gary L. Sanders, V.P. Engineering
Big...@Primenet.com
(Bob Nixon)
Gary L. Sanders
You bring up a sore point for hi-fi sound. Since the hi-fi tracks are
purely analog, there is a head switch point that occurs every field of
video, or 1/60 sec. Due to time-base error on the video and hi-fi output,
caused by tape stretch, friction, idlers, etc.; there is a glitch, and a
phase discontinuity at the head switch point.
The phase discontinuity is no issue at low audio frequencies, but as the
input audio frequencies get higher, the phase discontinuity is a greater
percentage of the wavelength, and VERY audible distortion results.
Try recording a sine-wave generator on your hi-fi machine and increase
the frequency in steps. The low frequencies sound great, but as you get
to 1kHz there starts to develop a 60 Hz buzz that beats with the audio
signal itself. By 2 or 3 kHz, it is very noticeable. Fortunately, on real
music, the sound is pretty good (I wonder if the golden-ears people would
notice?!). But it is a potential problem on some music.
Bob Nixon wrote:
>
> "Gary L. Sanders" <gls...@ix.netcom.com> wrote:
>
> Many thanks Gary. This is exactly what I was looking for. One
> more small question please. When the FM modulated audio is
> recorded on the diagonally scanning tape, how is the audio put
> back together during playback? It seems like audio unlike video
> is serial or continuous whereas video is a natural for this type
> of one frames information for each swipe across the tape.
> It were digital PCM I could see no problems but it's FM.
>
> Related issue:
> On poorly recorded tapes the sound track sometimes has a buzz
> (tracking related). Is this a failure to put this audio track
> (back together) properly?
>
> TIA
> Big...@Primenet.com
> (Bob Nixon)--
Gary L. Sanders, V.P. Engineering
pol...@hg.uleth.ca
I would like to supplement Gary Sanders post on VHS/SVHS modulation techniques
with some numerical data.
For recording, the NTSC luma in VHS systems is frequency modulated onto a
carrier centered at about 4 MHz, with the sync tip at 3.4 Mhz and the peak
white at 4.4 MHz, for a total deviation of 1 MHz. Of course, side bands are
produced which extend well beyond this range and contain a lot of useful
picture information. Care has to be taken to keep the lower luminance sideband
comparatively free of the "downconverted" (heterodyned) chroma upper sideband
which reaches out to 1.13 MHz from its center at 629 KHz. To this end a High
Pass Filter with a frequency cutoff (i.e. -3 dB attenuation) of 1.4 MHz was
typically used in older VHS machines.
In SVHS systems the luminance carrier has a deviation of 1.6 MHz, with a
sync tip of 5.4 MHz and peak white of 7 MHz.
Alexander the Great jr.
In article <3230D6...@ix.netcom.com>,
Gary L. Sanders <gls...@ix.netcom.com> wrote:
>You bring up a sore point for hi-fi sound. Since the hi-fi tracks are
>purely analog, there is a head switch point that occurs every field of
>video, or 1/60 sec. Due to time-base error on the video and hi-fi output,
>caused by tape stretch, friction, idlers, etc.; there is a glitch, and a
>phase discontinuity at the head switch point.
>
>The phase discontinuity is no issue at low audio frequencies, but as the
>input audio frequencies get higher, the phase discontinuity is a greater
>percentage of the wavelength, and VERY audible distortion results.
>
>Try recording a sine-wave generator on your hi-fi machine and increase
>the frequency in steps. The low frequencies sound great, but as you get
>to 1kHz there starts to develop a 60 Hz buzz that beats with the audio
>signal itself. By 2 or 3 kHz, it is very noticeable. Fortunately, on real
>music, the sound is pretty good (I wonder if the golden-ears people would
>notice?!). But it is a potential problem on some music.
The schematics of my S-VHS VCR indicate that a sample and hold circuit
is used to mask off the head switching point from the reproduced
VHS Hi-Fi audio. Needless to say, the frequency of repetition of
this sample and hold action is 60 Hz which explains nicely why
a 60 Hz frequency component is being added on any repetitive high
frequency waveform.
Has anyone noticed that although a 20-20K Hz frequency response
and a 90dB S/N ratio is touted for VHS Hi-Fi, the specs never
mention any of the finer details that would make audio really
Hi-Fi, for example THD?
By the way, the 90dB SNR is the result of audio compression.
I have no data on the details but I assume a 45dB intrinsic
SNR and a 2:1 compression is very likely.
Someday, if I have time to burn, I will record some sine-waves
and watch their playback on the scope. I bet I should use
TV-Vertical trigger to capture that!!!
I already know from tests I did in the past that the 60Hz S&H is
audible when recording certain sinusoidal frequencies. On the other
hand, I can attest as an audiophile and critical listener that it
is rarely audible when playing back music or speech.
I am speculating that there is some psychoacoustical masking
effect when there is sufficient energy in the low frequencies
(most movie soundtracks fall in this category)
and on top of that the 60Hz S&H is uncorrelated to the typical
musical waveform. Our ear and brain is "designed" to be sensitive
in detecting short bursts of noise in silence, but totally
insensitive in detecting bursts of silence in the noise!!!
<<alex kanaris>>
==============================================================================
Alexander Kanaris ___ ___ kan...@bode.usc.edu
Electrical Engineering --- Systems / / /__ / kan...@thales.usc.edu
University of Southern California /__/ ___/ /__ kan...@girtab.usc.edu
Los Angeles, California 90089-2562 kan...@alumni.caltech.edu
==============================================================================
--
->
Faith, n:
That quality which enables us to believe what we know to be untrue.
Gary L. Sanders
The sample and hold is really a switch that switches during the overlap
of the tracks, so that the sound is as continuous as possible. It is the
discontinuity at the time of the switching that causes the 60Hz
component. Distortion on sine waves at high frequencies is bad, that why
you don't see spec's. The THD spec's would look pretty good at 100 Hz.
--