30 the bell system technical journal, january 1951 



Over-all System Performance 



Various means are used to describe the over-all performance of a carrier 

 system such as type N. Subjective tests show, as in other carrier systems, 

 that noticeable deterioration in speech quality occurs when many links are 

 connected in tandem. Satisfactory conversation has been carried on between 

 Milwaukee and Madison, Wisconsin over nine such links, representing a 

 total circuit length of about 750 miles. In this circuit connection, speech 

 passed through 9 compandors, 108 group repeaters and 117 stages of modula- 

 tion. Practically all of the speech impairment occurred in the 9 compandors. 



100 

 90 



Z 70 



o 



to 

 52 60 



«0 



<50 

 cc 



»- 



a. 



Ui 



^30 



90 DBA 



82. 



72 



54 DBA 



+8VU (3<r LOUD TALKER) 

 OVU 



-10 VU (AVERAGE TALKER) 

 -28 VU (SdTWEAK TALKER) 



WORST NOISE DUE TO 

 REPEATER IN EXCHANGE 

 28DBA AREA OFFICE 



600~ TONE 



20 DBA (NOTED ^"'^'^.^vfi^|ffx '-^'*'^ 25 LINE NOISE TOLL , 



^ ■ ^-^ 18 DBA (NOTES) CONNECTION (NOTE 5) 



3700"^ TONE BATTERY CROSSTALK iP|rEPEATEr'1^0ISePJ> 



10 DBA (NOTE 2) 10 DBA (NOTE 4) J-'^^-'-i^'^V-^M^RT^ -^PM^^^-^^^ 



7 DBA 



— NOTES — 



1. BEATS BETWEEN LISTENING CHANNEL 3. +4VU INTERFERING TALKER. 



r^^T^^^liJS^,^ ^^^ "^"^^ °^ ^^^^~ 4. +4VU INTERFERING TALKER, 



CENT CHANNEL. p/^p END. 



2. SIGNALING TONE ON LISTENING CHANNEL. 5. 10-REPEATER SYSTEM 



Fig. 17 — Relative levels of speech and interference on N-1 carrier. 



When only six links were used (which is about the maximum likely to be 

 encountered in service) little impairment was observed. Generally even a 

 critical observer cannot distinguish between a single N channel link and 

 a direct circuit connection between the transmitter and the receiver of the 

 same noise and bandwidth. In Fig. 16, (a) and (b), the frequency character- 

 istic and limiting characteristic of the channel are shown. The useful band 

 of speech frequencies passed is considered to be between 10 db points in 

 four links or about 200 cycles to 3100 cycles. In the N system, because the 

 compandor control circuits are particularly wide-band, the frequency re- 

 sponses are substantially alike when measured with single frequencies or 

 when actuated by speech. 



