Modified electronic voice encoder /Vocoder/ includes an independent analog mode of operation in addition to the conventional digital mode. The Vocoder is a bandwidth compression equipment that permits voice transmission over channels, having only a fraction of the bandwidth required for conventional telephone-quality speech transmission

Abel, R. S.

Watkins, H. E.

English

NASA Technical Reports Server

December 1967	 .	 Brief 67-10571 
NASA TECH BRIEF 
__1w 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Analog Voicing Detector Responds to Pitch 
oicun I	 Hiss 
Circuit 
TX I I Generator 
I	 Pitch 1ii1 Buffer I I	 Pitch 
Extractor Pass IAnalof.i
-•i
And Level' I 
Filter L9.J
I
Shift Generator 
4 
oicin
Gate 
ANALYZER CHANNEL
Ir
SYNTH. CHANNE L 
-
- - - -
- -
-
---I 
Low I I
Mod
Amplifier	 u- B erluff B  
I.
Pass And
II!1Ttl i No. lator 
I
Pitch 
Speech	 In
Pamtm
Excitation 
Vogad Multiplex
LH
PCM Digital AMP1lReonst 
H
________
Dx Mux Voicing e-Emp Pre-Emph 
__________ 
__________  
Encoder PCM Out Decoder I Speech 
0 0 0 0	 0 
0 0 
E
LPF 
A2^i
 No. 16 Rect 22 Hz No.16 o.6 
- - - TRANSMITTER RECEIVER
An existing, commercially available electronic voice 
encoder (Vocoder) has been modified to include an 
independent analog mode of operation in addition to 
the conventional digital mode. The Vocoder is a band-
width compression equipment that permits voice 
transmission over channels having only a fraction of 
the bandwidth required for conventional telephone-
quality speech transmission. 
Referring to the diagram, the essential elements of 
a channel Vocoder consist of a voice operated gain 
adjustment (Vogad), a pitch extractor, and a bank of 
(N) contiguous bandpass filters each followed by an 
envelope detector. The Vogad maintains a relatively 
constant level to the analyzers regardless of the voice, 
and therefore, drives the analyzers at essentially the
same level for all speakers. The envelope detector in 
each analyzer channel provides a low frequency pa-
rameter, commonly limited to 22 Hz variations, repre-
senting the speech energy in that frequency band. In 
some channel Vocoders, there are 16 such frequency 
bands covering the speech frequency range from 200 
Hz to 4 kHz. 
The pitch extractor develops an analog voltage pro-
portional to the fundamental pitch frequency of the 
speech waveform during voiced sounds, this voltage 
being limited to variations at approximately the syl-
labic rate of 22 Hz. The total analog bandwidth is 
then 22 Hz x N+l parameters, or 374 Hz. In the 
receiver, the analog parameters are modulated by an 
excitation source in the form of a voltage-controlled 
(continued overleaf) 
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oscillator that is controlled by the pitch signal. Un-
voiced sounds are modulated by hiss which is gated 
in when the pitch signal is zero. The resulting sig-
nals are bandpass filtered in a set of contiguous 
filters, identical to those used in the analyzer, and 
then summed to form a facsimile of the original speech 
waveform. 
For nominal digital operation, the low frequency 
parameters are time-division multiplexed to form a 
pulse amplitude modulated (PAM) signal that is then 
encoded in a binary manner for transmission in a 
pulse code modulated (PCM) system. The N + l chan-
nel signals are sampled at 44.4 Hz to conform to the 
Nyquist sampling criterion and encoded in a 3-bit 
code. The pitch is encoded more accurately in a 6-bit 
code, resulting in a data rate of 2400 bps. Generally, 
one of the 16 channels is encoded in 2 bits and the 
remaining bit is used as a reference for synchroniza-
tion. A PCM decoder in the receiver converts the 
PCM back to PAM form which is demultiplexed into 
the proper channels.
Notes: 
1. Using a delta modulation technique, the Vocoder 
can operate at 1200 bps though there is a sacrifice 
in intelligibility and quality of the reconstructed 
signals. The PCM is readily adaptable to secure 
transmission; because of the nature of the signal, 
it is easily encrypted by combining with a pre-
selected digital key. 
2. Inquiries concerning this invention may be directed 
to:
Technology Utilization Officer 
Goddard Space Flight Center 
Greenbelt, Maryland 20771 
Reference: B67-10571 
Patent status: 
Inquiries about obtaining rights for the commer-
cial use of this invention may be made to NASA, 
Code OP, Washington, D.C. 20546. 
Source: Ronald S. Abel and Harold E. Watkins 
of Philco-Ford Corporation 
under contract to
Goddard Space Flight Center
(GSC- 10085) 
Brief 67-10571	 Category 01


Analog voicing detector responds to pitch

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