790 Federal Communications Commission Reports
BEFORE THE
FEDERAL COMMUNICATIONS COMMISSION
WASHINGTON, D.C. 20554
Dolby Encoders
July 10, 1974
THE COMMISSION ISSUED THE FOLLOWING PUBLIC NOTICE:
Commission Autlwrizes Use of Dolby Encoder by FM Broadcast
Stations
Under date of May 31, 1974, the Commission sent the following
letter to Dolby Laboratories, Inc.
"We have reviewed the question of the employment of the Dolby 324 B-Type
Broadcast Encoder by FM broadcast stations licensed by the Commission, and will
not object to its use.
"It is our understanding that, in the current model of the 324 encoder circuits
designed to reduce the effective degree of preemphasis to 25 microseconds, and
those producing Type B encoding operate simultaneously at all times-that it is
impossible, without internal changes, to utilize this apparatus in an operating mode
where only the preemphasis adjusting circuits are effective. Our concurrence in the
use of this encoder is given on the express condition that all units furnished to and
installed in FM broadcast stations are arranged to function in this manner.
"When FM broadcast stations conduct transmitter performance measurements
pursuant to Section 73.254 of our rules the Dolby 324 B-Type Broadcast Encoder
should be removed from the circuit, or effectively by-passed."
Some recent publicity concerning this action appears to reflect a
misunderstanding of its nature and effect. Accordingly, we believe a
clarification may be helpful.
First, it should be emphasized that we have not amended
§73.317(a)(2) of our rules, which requires the audio response of an FM
broadcast transmitter be shaped in accordance with a 75 microsecond
preemphasis curve. We have no present intention of doing so. Existing
receivers incorporate complementary 75 microsecond deemphasis cir 
cuits. Any substantial reduction in the amount of transmitter preem 
phasis, alone, would result in a more or less serious "roll off" in the
response of these receivers at higher audio frequencies.
Second, we are permitting, not requiring the use of the Dolby
encoder. In this respect our policy is the same as that which has applied
heretofore with respect to the use of other kinds of active audio I:'...·....
processing equipment, such as the same type of limitations on Dolby
employed by FM stations.
The Dolby unit which we have indicated as acceptable for use by FM
broadcast stations incorporates circuitry which, in effect, cancels a
72 F.e.e. 2d
Reregulatwn of Radw and TV Broadcasting 791
portion of the transmitter preemphasis. However, the unit is so
constructed that this circuitry is effective only when Type 3 encoding
is employed. The encoding process raises the level of the higher audio
frequencies relative to lower frequencies by an amount which varies
inversely with the average level of the audio signal, the maximum
"boost" occurring at low modulation levels, with virtually none at high
levels of modulation. While this process, therefore, does not fully
compensate for the lower effective degree of fixed preemphasis
employed, we have satisfied ourselves that this combination of
techniques results in a transmitted signal which will be reproduced by
a conventional receiver with no apparent loss in high frequency
response.
We have not determined, nor do we believe, that a listener to a
Dolby equipped FM station would enjoy fully compatible reception if
he were to modify his FM receiver solely by changing the amount of
deemphasis from·75 to 25 microseconds.
Improvements in signal-to-noise ratio attained through the use of
Dolby equipment should be apparent to listeners in areas where the
received signal is comparatively weak. Such areas are generally quite
distant from the station being received. The maximum improvement in
signal-to-noise ratio can be achieved only when the receiver is equipped
with complementary Dolby circuitry. However, the carrier of an FM
station with the Type 324 encoder, with the lower effective degree of
preemphasis which its use entails, may be modulated at a higher
average level than would otherwise be the case. When this is done, the
noise performance of conventional receivers tuned to the station will
improve.
Dolby techniques operate only to reduce the effect of inherent noise
developed within the FM receiver. In distant areas where such a
reduction might otherwise be useful, the limitation on the ability to
receive a particular FM station satisfactorily may be imposed not by
receiver noise, but by interference from other co-channel or adjacent
channel FM stations.
72 F.C.C. 2d