Monday, December 3, 2012

Remembering The Dolby Encoded Stereo FM Broadcasts And The Cassette Tape Deck Hi-Fi System

Working musicians back then often cringe at the idea, but do older hi-fi enthusiasts still remember the time when the hi-fi cassette tape deck and the FM tuner worked as a complete “recording system”?

By: Ringo Bones

Working musicians back then whose livelihoods are more or less solely dependent on the revenue of the sales of their copyrighted works often frown upon the prospect of the masses relying as their main recorded music source off-air recording from FM stereo broadcasts. Worse still, near the end of the 1970s, a number of FM stations being run by broadcasters / engineers with “Golden Ears” started to broadcast their music programs in a format called “Dolbyized FM” or “Dolby-Encoded FM broadcasts” where a hi-fi enthusiast with a well-aligned and set-up hi-fi cassette tape deck could “potentially” make off-air recordings whose sound quality that’s as good as or even better – and judging by my first hand experience of finding such cassette tapes in recent garage sales and swap meets often way better – than prerecorded music cassette tapes sold by major labels in licensed music stores. Even though no “integration” yet exists between the i-Pod and the “legal and licensed” online music stores selling downloadable digital music in the form of preemphasis for jitter reduction and what have you – was the “integration” between the hi-fi FM tuner and the hi-fi cassette tape deck via Dolby-encoded FM stereo broadcasts the only time in history where there is system integration between broadcasting and home recording?

Since big-wig engineers running those online “licensed and legal” digital music downloads seems to be sitting on their asses when it comes to pushing for better sound quality, I’ll just reminisce about the good old days of using your hi-fi cassette tape deck to record Dolbyized FM broadcasts. By the late 1970s, “affordable” hi-fi cassette tape decks – if you consider 400 US dollars affordable back in 1979 – started to enter the market that can integrate with FM stations providing Dolby-encoded broadcasting service. Usually in its Dolby noise selector switch - usually with the 19-KHz pilot tone filter for the cassette tape deck's built-in Dolby noise reduction system to work properly with Dolbyized FM stereo broadcasts, there is a position labeled FM which is used to process audio from an FM station that Dolby-encodes its programs. In this mode, recording-level controls are disabled and the input level is controlled usually by two screwdriver controls – on the rear apron – using the test tones that are periodically transmitted (usually in the wee hours of the morning an hour prior to their regularly-scheduled broadcasts) by FM stations that use Dolby processing. The FM mode also converts the 75-microsecond deemphasis of the tuner’s output to the 25-microsecond required by the Dolby noise-reduction system.

Cassette tape decks that are designed to be integrated with Dolby-encoded FM broadcasts are usually equipped with recording heads that are an engineering tour-de-force in comparison to their early 1990s era siblings because cassette tapes recording heads used in such machines back then had to be driven to about +10dB with most tapes before third-harmonic tape distortion reached the reference 3 percent level. While ones made by leading brands – like Sony, Nakamichi and Teac just to name a few -  had to be driven to +13dB before the 3 percent third-harmonic distortion mark is reached. Recording heads used in “affordable” cassette tape decks made around 1994 or later started to reach the 3 percent third-harmonic distortion mark if the signal reaches +4dB – even with metal tapes!

Back in the late 1970s and early 1980s, a good cassette tape deck that is designed to cope with the likely “excesses” of the Dolby-encoded FM broadcasting system usually have a headroom of 7 to 10dB beyond the Dolby level before distortion reaches 3 percent. Therefore, Dolby-encoded FM programs can be recorded with fully effective Dolby operation with no risk of tape saturation and the resulting loss of high frequency signals and increased distortion. And their VU meters are accurate to within 1dB of the correct 200 nW/m flux level for a standard Dolby tape.

On most recorders back then, if the FM Dolby signal levels are adjusted correctly for the Dolby system, with 50 percent modulation corresponding to a Dolby-level meter indication on the recorder, 100 percent modulated peaks will be at +6dB and will almost certainly overload the recorder. The only alternative in most cases is to set the Dolby tone from the FM transmitter (50 percent modulation) several decibels below the meters’ Dolby points, which can degrade frequency response and noise reduction but will not distort. These examples are based on recording the signal without decoding – a theoretically preferable approach. Often, the easiest solution is to decode the Dolby-encoded signal and record it in that form at correct levels.