Resolution can be obtained at large compression ratios with method for selecting data points for transmission by telemetry in television compressed-data system. Test slope of raw data stream and compare it to symmetric pair of decreasing thresholds. When either threshold is exceeded, data are sampled and transmitted; thresholds are reset, and test begins again

Kleinrock, L.

English

NASA Technical Reports Server

,ft 
. 0 
0	 SAMPLE 
-	 RECONSTRU 
FUNCTION 
• THRESHOLD 
REGION 
Af,,	 \
September 1973	 B73-10382 
S NASA TECH BRIEF 
NASA Pasadena Office	
-4r 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Data Compression by a Decreasing Slope-Threshold Test 
Excellent resolution can be obtained at large 
compression ratios with a novel method for selecting 
data points for transmission by telemetry in a tele-
vision (TV) compressed-data system. A net compres-
sion ratio of 3 or 4 to 1 is achievable with no visually 
detectable degradation in TV picture quality. 
The scheme of the slope-threshold compression 
method is to test the slope (first difference) of a raw 
data stream and compare it to a symmetric pair of 
decreasing thresholds. When either threshold is ex-
ceeded, the data are sampled and transmitted; the 
thresholds are reset, and the test begins again, as 
follows: 
Let	 f = raw data sequence 
= f—f_ 
nm = index of the mU transmitted sample.
When the M1h sample has already been transmitted 
and the search for the m+1 sample is in progress, 
the upper and lower thresholds Tu(n) and TL(n), 
are (for a >0, b>0) 
Tj(n) = Af,, + a exp[—b(n—n.-1)] 
TL(n) = Afr,	 a exp[—b(n—nm—i)]. 
Thus, when M—Tu(n) ^!0, or when M—TL(n) :!^0, 
f.—i is sampled and nm+1 is set equal to n—i. The 
reconstructed function is a linear interpolation of 
these samples. Referring to the diagram, point 6 is 
not within the threshold; therefore, point 5 would be 
sampled. Point 7 is not within the threshold; there-
fore, point 6 would be sampled. Point 8 is within; 
therefore, point 7 would not be sampled. Point 9 is 
within; therefore, point 8 would not be sampled. 
Point 10 is not within; therefore, point 9 would be 
sampled. 
The decreasing slope-threshold method is easily 
implemented in analog form as shown in the circuit 
diagram. The raw data sequence f is applied to the 
input of the decaying slope-threshold sampling cir-
cuit and also to the adder (operational amplifier) and 
unit delay line. The output of the unit delay line is 
the raw data sequence f having an index n—i. To 
obtain the change, Af., in the data, the raw data 
sequence fn is algebraically summed with f,j in the 
adder. The unit delayed raw data sequence is 
also applied to an AND gate which will gate f_1 to 
the output when a coincident sampling command is 
received from the sample circuit (which is a one-shot 
multivibrator). 
The difference voltageAfn is applied to adders in 
opposite polarities where it is algebraically summed 
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights. 
https://ntrs.nasa.gov/search.jsp?R=19730000382 2020-03-17T07:03:49+00:00Z
Sample 
To _	 ____ 
Telemetry
Unit 
Delay
- 
+
Compare I 	 o 
to  +
Jr 
Sample 7^ 
D	 ___  
From Data	
-
Set	 I ri 
Processors	 + 11+ and Hold	 1+ •	 I 
a 1 
Af
_I	 T -	 I ii TL	 b 
- 
-- - - -	
- -
•	
Compare 
j 
O 
to 
. 
with the upper and lower decaying thresholds em-
anating from the biased, decaying threshold-gener-
ating circuit. This circuit generates the decaying 
voltage
a exp [b(flflm1)1 + M 
where a is the dc standard voltage source and a exp 
1b(flflm1)1 is the capacitor charge decay due to 
the time constant of the RC circuit. The start of the 
decaying threshold is coincident with the sampling 
pulse by the operation of an AND gate. A bias is 
derived from the fact that the voltage Afn is gated 
to adders by operation of an AND gate and the 
HOLD circuit (capacitor storage). The bias centers 
the decaying thresholds above and below the pre-
vious sampled point. 
The resulti of the subtractions in the first adders 
determine if M—T(n) 0, or if M,,-TL(n)<O. If the 
remainder is zero outside of the upper Tu(n) and 
lower TL(n) thresholds, the delayed data point must 
be sampled. Therefore, an output from either of the
adders would trigger the sample circuit by operation 
of Schmitt trigger circuits. The f_ 1 data point would 
thereupon be gated to the output and would be one 
point that would be transmitted by telemetry in the 
compressed data channel. 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,598,921). Inquiries concerning nonex-
clusive or exclusive license for its commercial devel-
opment should be addressed to: 
NASA Patent Counsel 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Source: Leonard Kleinrock of 
Caltech/JPL
under contract to 
NASA Pasadena Office 
(NPO-10769) 
B73-10382	 Category 02 


Data compression by a decreasing slope-threshold test

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