Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site hydra.UUCP Path: utzoo!linus!philabs!cmcl2!seismo!harvard!godot!mit-eddie!cybvax0!frog!hydra!admin From: admin@hydra.UUCP (admin) Newsgroups: net.video,net.analog Subject: How does Beta HiFi audio avoid head switching transients Message-ID: <116@hydra.UUCP> Date: Sun, 10-Mar-85 00:00:52 EST Article-I.D.: hydra.116 Posted: Sun Mar 10 00:00:52 1985 Date-Received: Tue, 12-Mar-85 20:29:32 EST Distribution: net Organization: Charles River Data Systems, Framingham MA Lines: 52 Xref: linus net.video:924 net.analog:180 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 ?