lionel@eiffel.DEC (Steve Lionel) (03/13/85)
From my Sony SL-2700 "Operations Manual" (a service manual) comes the
following description of the Beta Hi-Fi recording method:
The Beta hi-fi system also FM-modulates audio signals (A-FM signal) and
records them on the video track on which only video signals have been
recorded in the past.
To prevent crosstalk between tracks, the BETA HIFI System uses two
carriers for voice signals for one channel using the A ch video head
and B ch video head. Four carriers are needed for two channels.
Note: In BIII, the head width is 33um [finally, an answer to this! - SL]
compared with a track width of 19.5um, and 1/3 of the head covers the
adjoining channel. The frequency of the A-FM signals is lower than Y
signals, and head azimuth losses are negligible.
These A-FM signals are inserted every 150 kHz in the 600kHz band between
the low frequency conversion chroma signals (688 kHz) and the Y-FM
signals. The frequency of each A-FM signal carrier is interleaved on the
playback pictures with the chroma and Y signal systems.
There are no problems with compatibility with conventional models regarding
the Y signal output level due to the slightly upward change in the Y-FM
signal frequency, as the frequency deviation of the Y-FM signals is the same
as before, 1.2MHz.
Carrier Frequency* Video Track Video Channel
f1 1.380682MHz A L
f2 1.530157MHz B L
f3 1.679633MHz A R
f4 1.829108MHz B R
* Interleaving is performed by changing the A-FM
signal carrier frequency to 1/4 (2n + 1) fH
(n=1,2,3) [fH is ?] For this reason, interference
(from cross-modulation between A-FM and Y-FM
signals) on the reproduced pictures from beating
caused by the non-linearity of the transmission
system (tape magnetic characteristics, etc.) can
be eliminated.
[Did they perhaps mean Audio Channel instead of Video
Channel above? This manual contains a fair amount of
Japanglish - SL]
The maximum frequency deviation of the A-FM signals is +/-75kHz
as in FM broadcasting, and intervals between carriers are 150kHz
(9.5 fH). [Perhaps fH is horizontal oscillator frequency - SL]
Carriers for A and B tracks are inserted alternately.
The mutual frequency difference is stringently controlled for the
following reasons. Both f1 and f2 are L ch voice signals and are
FM modulated. However, when the mutual frequency difference
deviates, this shows up as a difference in the DC level during
FM demodulation. Because of this, a 30 Hz noise step appears in
L ch voice signals obtained by switching f1 and f2 demodulated
outputs with an RF switch pulse. This also applies to f3 and f4.
The specified limit to the frequency deviation between f1 and
f3 is within 10kHz, respectively. To allow interleaving, the
deviation should be approximately 3kHz.
The maximum frequency deviation and standard modulation level of
A-FM signal are controlled as shown here.
Depth of Frequency
Modulation Deviation
Maximum Frequency
Deviation 100% +/- 75kHz
Standard Modulation
Level (0 db) 33% +/- 25kHz
Interference is caused to adjacent A-FM signals when the depth of
modulation exceeds 100%. In reality, the maximum depth of
modulation should b held to about 92% considering the BPF
characteristics [?].
[What follows are excerpts from more text, leaving out the circuit details.]
The chroma signal system does not differ greatly between the A-FM
and conventional systems. However, the Y signal system has the following
three differences.
1. The frequency deviation of the FM modulator is shifted
slightly higher.
2. A filter is needed before mixing the Y-FM and A-FM signals
during recording to remove that part of the Y-FM band
corresponding to the A-FM signal band.
3. A filter to remove the A-FM signals is needed before inputting
Y-FM signals to the FM demodulator during playback.
Low frequency signals are companded and given dynamic pre-emphasis. The
signals are fed to a limiter to prevent overmodulation and are held
below 100% depth of modulation.
A ch is recorded with f1 modulated by the L ch voice frequency and f3
modulated by the R ch voice frequency. B ch is recorded with f2 modulated
by the L ch voice frequency and f4 modulated by the R ch voice frequency.
f1 is translated to f2 in the BM (Balanced Modulator), and f3 is
translated to f4 in the BM. Both are input to the demodulators. Switching
from A ch to B ch is performed by an RF SW pulse. The FM waves are demodulated
by the demodulator to the voice band. Holds 1 and 2 remove noise during
changeover from A ch to B ch. Hold 2 holds the carrier during drop-out to
prevent sound or shift. The expander also performs dynamic de-emphasis
to give a low frequency output matching the input.
[So, I guess the answer to the question as to how Beta Hi-Fi avoids noise
during head switches is that it has circuits to hold the signal constant
during the changeovers. Perhaps someone who understands this mumbo-jumbo
better than I can interpret better.]
Steve Lionel