[sci.electronics] Digital recording on a VCR

simmonds@demon.siemens.com (Tom Simmonds) (01/16/90)

I would like to build a circuit to digitize an audio signal, possibly in
PCM form, and record it on my VCR, and also to play it back.  The video
bandwidth should make it easy, but I don't know enough about video signals
and VCR's.  Does anyone know what I have to do to record a PCM signal on
a VCR?  Specifically, do I have to add a synch pulse in order to make it
record, or can I just take the PCM signal and put it on some carrier frequency
(channel 3, for instance) and feed that directly into the VCR without
simulating NTSC format?  Is there anything, besides the synch pulse, that
I have to do to the signal to make it record?

Thanks, in advance, for any help you may be able to offer.  Please email
your response to:  simmonds@demon.siemens.com

If I get any useful answers, I'll post a summary.


==================================== Tom Simmonds ============================

sorka@ucscb.UCSC.EDU (Alan Waterman) (01/17/90)

Several companies make a product that does what you want. It made vcr's
the DAT before DAT.

toddpw@tybalt.caltech.edu (Todd P. Whitesel) (01/18/90)

speaking of which, does anyone know what the restrictions on vcr video
signals are? besides sync, is there a voltage limit (i suppose there is one
because the line in/out says 1.0v peak-to-peak on everything i have).

me and a friend have wondered how tough it would be to record a stream of
digital pulses on a vcr tape and play them back (i.e. cheap HD backup, speed
doesn't really matter). We tried putting the serial port though an op-amp
and recording it on cassette tape, however we had some trouble reading the
whole text file back reliably. are there any bizarre restrictions on the
inputs to a tape recorder?

Todd Whitesel
toddpw @ tybalt.caltech.edu

henry@utzoo.uucp (Henry Spencer) (01/19/90)

In article <13400@cit-vax.Caltech.Edu> toddpw@tybalt.caltech.edu (Todd P. Whitesel) writes:
>me and a friend have wondered how tough it would be to record a stream of
>digital pulses on a vcr tape and play them back (i.e. cheap HD backup, speed
>doesn't really matter). We tried putting the serial port though an op-amp
>and recording it on cassette tape, however we had some trouble...

VCRs are not like audio recorders; they know that their input is a video
signal and tinker accordingly.  (For example, as I recall, they record sound
and picture separately on the tape, meaning they know how to take a video
signal apart.)  You're going to have to make your digital data look like
video, at least to the extent of having sync pulses, I'd guess.
-- 
1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu

phupp@warwick.ac.uk (S Millington) (01/19/90)

  Devices which provide HD backup on standard VCR's are available, in the
UK at least - I think I also saw an add in Byte so probably in the US aswell.
As far as I remember they put a few hundred megabytes on a short tape and cost
about 200 pounds. Unfortunately the only versions I've seen were with PC Card
interfaces.

Stuart Millington.
phupp@poppy.

maj@hpfinote.HP.COM (Mike Jassowski) (01/20/90)

/ hpfinote:sci.electronics / henry@utzoo.uucp (Henry Spencer) /  9:46 am  Jan 18, 1990 /
>In article <13400@cit-vax.Caltech.Edu> toddpw@tybalt.caltech.edu (Todd P. Whitesel) writes:
>>me and a friend have wondered how tough it would be to record a stream of
>>digital pulses on a vcr tape and play them back (i.e. cheap HD backup, speed
>>doesn't really matter). We tried putting the serial port though an op-amp
>>and recording it on cassette tape, however we had some trouble...

I had some luck recording digital on audio tapes.  I had to modulate the
digital information.  Audio recorders have trouble with DC; if you have 
several ones/zeros in a row, it starts to look like DC.  A little filtering
the output restored the informtion.
>
>VCRs are not like audio recorders; they know that their input is a video
>signal and tinker accordingly.  (For example, as I recall, they record sound
>and picture separately on the tape, meaning they know how to take a video
>signal apart.)  You're going to have to make your digital data look like
>video, at least to the extent of having sync pulses, I'd guess.

Most VCRs have separate "video in" and "audio in" ports, as well as the
RF connection.  If you used these, there may be less artificial video
stuff that needs to be done.  My thesis advisor claimed that an audio
type signal into the video port should work, but you would still have to
modulate any digital information (although I never tried it).  To give
this a try, you might just attempt an audio recording through the video
input, and see if it is distorted on playback.
>-- 
>1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
>1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
----------

john@frog.UUCP (John Woods) (01/20/90)

In article <1990Jan18.164648.15896@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) writes:
> VCRs are not like audio recorders; they know that their input is a video
> signal and tinker accordingly.  (For example, as I recall, they record sound
> and picture separately on the tape, meaning they know how to take a video
> signal apart.)  You're going to have to make your digital data look like
> video, at least to the extent of having sync pulses, I'd guess.

As I understand it, this is roughly how it *is* done: you make a TV picture
where the image is something that you can tear apart digitally (shades of the
old CRT computer memories !-)  For a while, WGBX TV Boston was running an
experimental encoding system where they would transmit digital audio in the
picture frames, with the audio from WGBH FM for reference in the audio channel.
You could actually SEE the bits.  Very entertaining watching (for a few seconds
anyway).
-- 
John Woods, Charles River Data Systems, Framingham MA, (508) 626-1101
...!decvax!frog!john, john@frog.UUCP, ...!mit-eddie!jfw, jfw@eddie.mit.edu

Happiness is Planet Earth in your rear-view mirror.	- Sam Hurt

wiml@blake.acs.washington.edu (William Lewis) (01/30/90)

   On the subject of recording info onto an audio tape, has anyone
tried using a modem to recored onto a cassette? My "original Hayes" had
some pretty odd commands, designed for use in radioteletype and such,
that should make it quite easy to do this. (Alas, my newer modem
has twice the speed but 1/100 the nifty options. "Hayes compatible",
my foot. Did you know a Hayes has special commands for sending
Morse code?) 
    On the other hand, modems are designed for the comparitively
narrow bandwidth of a phone line, so it should be possible to get
higher data rates on a VCR tape, even without all the fancy 0.0001
degree phase encoding ...

-- 
wiml@blake.acs.washington.edu        (206)526-5885      Seattle, Washington

strong@tc.fluke.COM (Norm Strong) (01/31/90)

In article <15880005@hpfinote.HP.COM> maj@hpfinote.HP.COM (Mike Jassowski) writes:
}/ hpfinote:sci.electronics / henry@utzoo.uucp (Henry Spencer) /  9:46 am  Jan 18, 1990 /
}>In article <13400@cit-vax.Caltech.Edu> toddpw@tybalt.caltech.edu (Todd P. Whitesel) writes:
}>>me and a friend have wondered how tough it would be to record a stream of
}>>digital pulses on a vcr tape and play them back (i.e. cheap HD backup, speed
}>>doesn't really matter). We tried putting the serial port though an op-amp
}>>and recording it on cassette tape, however we had some trouble...
}
}I had some luck recording digital on audio tapes.  I had to modulate the
}digital information.  Audio recorders have trouble with DC; if you have 
}several ones/zeros in a row, it starts to look like DC.  A little filtering
}the output restored the informtion.
}>
}>VCRs are not like audio recorders; they know that their input is a video
}>signal and tinker accordingly.  (For example, as I recall, they record sound

You could buy a Toshiba DX-900.  It has built in digital audio
capability.
-- 

Norm   (strong@tc.fluke.com)

annala@neuro.usc.edu (A J Annala) (02/11/90)

In article <13400@cit-vax.Caltech.Edu> toddpw@tybalt.caltech.edu (Todd P. Whitesel) writes:
>
>me and a friend have wondered how tough it would be to record a stream of
>digital pulses on a vcr tape and play them back (i.e. cheap HD backup, speed
>doesn't really matter). 

I have an eight channel sequentially multiplexed analog-to-digital converter
in my lab hooked up to a SONY PCM-F1 digital audio signal processor.  This
unit converts 16 bit ADC samples at 88.2 KHz and stores them on just about
any cheap commercially available low end consumer grade VCR.  It works just
as well in extended play mode as it does at higher speeds.  The SONY PCM-F1
generates a standard digital recording format called EIAJ PCM.  

The trouble comes when you try to hook this device up to an IBM PC/AT.  My
simple minded 16 bit wide synchronously clocked digital input board talks
to channel 6 of the second 8237A DMA chip just fine.  However, I am unable
to write any data to the hard disk (via turbo/microsoft c io library calls,
msdos file services, or direct calls to the hard disk read/write bios ints.
Does anyone have a clue why a DMA interface accepting 16 bit wide data at
88.1 KHz would interfere with access to my hard disk?  Does the controller
for the hard disk assume no dma cycles are present on the bus while it is
trying to write data to the disk?  Is there any solution that would allow
simultaneous DMA input and file oriented or raw hard disk output?

The only solution I have at the moment is to build a buffered digital
interface containing a FIFO chip (IDT7202 -- any know what this is --
where to buy a few of them -- where to get a spec sheet -- how large a
buffer memory is available) that will hold data while I mask (disable)
DMA input long enough to write to my hard disk.  This buffering scheme
was presented (but only briefly discussed) in Dr. Dobb's Journal Jan90
pp. 28-37, pp.94-95 with a hardware outline and supporting MSC5 code.
Does anyone make clocked digital input boards with large fifo buffers
for commercial purchase?

Thanks, AJ Annala, USC Neuroscience Program, HNB-534A, LA, CA 90089-2520
Telephone:  (213)743-3251 (office)  (213)743-8868 (lab)

-------------------------------------------------------------------------
For further information on constructing multi channel analog/digital to
VCR adaptors the following articles are quite helpful and detailed:

Bezanilla, Francisco.  A high capacity data recording device based on
a digital audio processor and a video cassette recorder.  Biophys. J.
47:437-442 (1985).

Lamb, T. D.  An inexpensive digital tape recorder suitable for
neurophysiological signals.  Journal of Neuroscience Methods 15:1-13
(1985).

Chiang, R. G. et al.  Recording electrophysiological data on video tape:
a superior and less costly alternative to conventional tape recorders.
Journal of Neuroscience Methods 15:15-20 (1985).

Blaettler, Markus et al.  Data recording and playback on video tape -- a
multi-channel analog interface for a digital audio processor system.  J.
Neuroscience Methods 23:139-148 (1988).

These systems are also available commercially from a number of sources.

Medical Systems Corp, One Plaza Road, Greenvale, NY 11548 (516)621-9190.

Vetter Instruments ... your neuroscience people get advertisements from
these people all of the time ... some of their adaptors are just simple
FM modulated carriers representing analog signals on modified VCR's ...
other adaptors are analog to digital --> multiplexed --> digital to PCM
--> recorded as bright spots on VCR.