nzm10@amdahl.UUCP (Neal Macklin) (10/23/85)
Does anyone know if a non-MTS-decoding VCR records the whole MTS signal
on the tape, or does it strip it off and only record the mono? In other
words, if you played the tape through an MTS decoding TV, would you hear
stereo? This question should have no bearing on if the VCR is a HIFI
type since HIFI would not be recorded in this case.
If I don't hear back at least from Mr. Video, I will be disappointed.
--
Neal Macklin
(408) 737-5214
...{hplabs,ihnp4}!amdahl!nzm10
[There are no opinions expressed in this article].die@hydra.UUCP (Dave Emery) (10/26/85)
In article <2136@amdahl.UUCP> nzm10@amdahl.UUCP (Neal Macklin) writes: >Does anyone know if a non-MTS-decoding VCR records the whole MTS signal >on the tape, or does it strip it off and only record the mono? In other >words, if you played the tape through an MTS decoding TV, would you hear >stereo? This question should have no bearing on if the VCR is a HIFI >type since HIFI would not be recorded in this case. Both HiFi and linear recording low pass filter the audio before recording it so the ultrasonic MTS subcarrier energy is stripped off the signal before it gets recorded. This means that what gets recorded on the tape has no stereo information and will thus reproduce as mono when played back through a MTS decoding TV. And the tape recording process is such as to hopelessly distort what MTS signal does sneak past the filters. In the case of linear recording (the low-fi direct recording by an audio head along the edge of the tape) the speed of the tape and head parameters are such that very little signal above about 10-12 khz ever gets recorded anyway, but even so record electronics may filter out ultrasonic components in the signal to be recorded to prevent beating phenomena with the ultrasonic bias signal applied to the head to improve signal to noise and distortion. (I suspect that many VCR's use a harmonic of 15734 hz (the horizontal scanning frequency) as bias to reduce beats with stray harmonics of 15734 that bleed into the audio but I don't have schematics handy to check). Linear recording at the tape speeds involved cannot come close to reproducing the stereo signal which would require a record channel flat to about 50 khz with very low phase shift at 32 khz (where the DSBSC L-R difference signal lives) relative to the 15.734 khz pilot and a variety of other difficult to meet specs as well. The fm recording by the spinning heads at high head to tape speed used in the Beta and VHS HIFI systems to record the hifi sound could in theory reproduce the frequencies used by MTS but in order to prevent problems with interference between the various subcarriers involved audio is low pass filtered before being applied to the vcos. [Even though the bandwidth is there the phase jitter due to head switching would have to be corrected before helical scan recording could be used to record a quality MTS signal} I suspect that some VCR's that use dolby B or other frequency sensitive companding schemes on their linear sound tracks may actually incorperate notch filters at the 15734 hz pilot frequency to prevent pilot energy (or other horizontal frequency energy such as leakage from the video or scrambling system pilots) from interfering with the proper operation of the companders. And finally, the sound demodulator in a VCR applies deemphasis to the audio signal demodulated from the fm sound carrier before supplying it to the record electronics because the fm sound on a TV station is preemphasised to improve snr at high frequencies. The deemphasis network acts like a low pass filter and knocks out much of the MTS subcarrier energy before it even reaches the record electronics. So, in summary, the only way to record stereo tv in stereo is to use a MTS decoder to decode the signal and record the L + R signals separately. VCR's cannot reproduce the MTS signal. David I. Emery Charles River Data Systems 617-626-1102 983 Concord St., Framingham, MA 01701. uucp: decvax!frog!die