Page "VHS" Paragraph 39

Around 1984, JVC added Hi-Fi audio to VHS ( in response to Betamax's introduction of Beta Hi-Fi.

) Both VHS Hi-Fi and Betamax Hi-Fi delivered flat full-range frequency response ( 20 Hz to 20 kHz ), excellent 70 dB signal-to-noise ratio ( in consumer space, second only to the compact disc ), dynamic range of 90 dB, and professional audio-grade channel separation ( more than 70dB ).

VHS Hi-Fi audio is achieved by using audio frequency modulation ( AFM ), modulating the two stereo channels ( L, R ) on two different frequency-modulated carriers and embedding the combined modulated audio signal pair into the video signal.

To avoid crosstalk and interference from the primary video carrier, VHS's implementation of AFM relied on a form of magnetic recording called depth multiplexing.

The modulated audio carrier pair was placed in the hitherto-unused frequency range between the luminance and the color carrier ( below 1. 6 MHz ), and recorded first.

Subsequently, the video head erases and re-records the video signal ( combined luminance and color signal ) over the same tape surface, but the video signal's higher center frequency results in a shallower magnetization of the tape, allowing both the video and residual AFM audio signal to coexist on tape.

( PAL versions of Beta Hi-Fi use this same technique ).

During playback, VHS Hi-Fi recovers the depth-recorded AFM signal by subtracting the audio head's signal ( which contains the AFM signal contaminated by a weak image of the video signal ) from the video head's signal ( which contains only the video signal ), then demodulates the left and right audio channels from their respective frequency carriers.

The end result of the complex process was audio of outstanding fidelity, which was uniformly solid across all tape-speeds ( EP, LP or SP.

) Since JVC had gone through the complexity of ensuring Hi-Fi's backward compatibility with non-Hi-Fi VCRs, virtually all studio home video releases produced after this time contained Hi-Fi audio tracks, in addition to the linear audio track.

Under normal circumstances, all Hi-Fi VHS VCRs will record Hi-Fi and linear audio simultaneously to ensure compatibility with VCRs without Hi-Fi playback, though only early high-end Hi-Fi machines provided linear stereo compatibility.