14()6 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



The tabulation is first given for an average channel. The performance 

 of the worst channel has, however, also been included to give an indica- 

 tion of its contribution to over-all operation. 



Many of the Nl carrier telephone circuits in the plant show lower 

 noise than the objective, and to this extent Table III is somewhat 

 pessimistic. Also some of the tests have shown, particularly for AM, 

 that the performance is somewhat improved by removing the com- 

 pandors. Thus, allowing for both points, better performance can be 

 expected from the average Nl circuit (less than 200 miles) in the plant. 

 The transmission coefficient of 11.5 listed for Item 6 at 100 words per 

 minute might go down to say 9. At 75 words per minute with FS or 

 AM, it might not be over 4.5. It is clear, of course, that with further 

 modifications of the Nl channels, such as to reduce noise exposures, 

 better performance could be obtained. 



The broad conclusions that can be derived from these considerations 

 are: 



1. The subdivision of the frec}uency band into telegraph channels, 

 and the use of FS, permit a workable system to be operated over a 

 compandored facility like the Nl carrier without modification. This 

 occurs even when the latter has noise up to and a little over the tele- 

 phone objective. 



2. This workable system under such noisy conditions transmits up 

 to some 350 bits per second with AM, and some 800 bits per second 

 with FS. It is accomplished with an error rate of the order that has 

 been implied for data transmission, even in the worst channel. 



3. There is a relatively wide range of performance of the system over 

 different Nl circuits, and the average performance is sensibly better 

 than that under the limiting conditions which have been considered. 



2.4.2 Distribution of Signal in Allocated Bandwidth 



A more extensive discussion of the use of bandwidth is given in Sec- 

 tion 3.1, below. However, a few specific points are appropriate here on 

 the band use in telegraph channels. 



The spectrum of the original voice frequency telegraph system, based 



Table IV — Use of Frequency Spectrum in Telegraph Channel 



