[BC] Xmitters at aol.com

Tue Apr 10 09:41:32 CDT 2007

Just because they do it, does not mean it's correct. Check out  http://en.wikipedia.org/wiki/WCRB . I was that WCRB Engineer in the very early days of FM Stereo Broadcasting, that spent a lot of time getting it right. Early on, there was no question that the repeat coils should be connected 1:1, 500 ohms to 500 ohms. That was the default strapping and telco didn't provide a schematic. However, attempts to phase/frequency match stereo pairs were not always successful. Therefore, I discovered that it was possible to "cheat" and strap the studio-end (only) for 2:1 and improve the frequency response for some lines just enough to pass a proof-of-performance.

Here is the relevent text from Wikipedia:

"--- Other technical innovations followed. Before the early days of FM stereo broadcasting, nobody had encountered the necessity of amplitude and phase-matching two 15 kHz stereo-leased lines. The telephone company called such a channel type; “Program channel A.” To them, as long as the frequency response and noise level matched their specifications, stereo simply meant that there would be two lines. It was just a matter of labeling them! Not so. WCRB engineering worked with AT&T to generate a specification involving matching both the phase and frequency response. This became the standard of the industry. Eventually, as stereo caught on across the country, these methods and specifications were used to install stereophonic leased lines to transmitters across the country until they were made obsolete by the development of composite-signal studio to transmitter links. In the early days of radio, stations had full-time engineers on duty.---"

In the meantime, if these lessons taught were corrupted to where all installations feed the telco circuits with 1/2 the voltage allowed, then this system hurts the available S/N ratio. Again, the 150 ohm strapping should be used only where necessary to help fix a frequency-response problem.

--
Cheers,
Richard B. Johnson
Read about my book
http://www.AbominableFirebug.com

 -------------- Original message ----------------------
From: dhultsman5 at aol.com
> Richard:
>  
> I have seen a many equalized telephone line, 5, 8, 15 kHz. (Audio) response  
> lines equalized by telephone techs and in all cases the line side was strapped 
> 4:1 at the transmitt end and 125 ohms at the receive end with the equalizer 
> strapped across t & R.    I have not been involved with the newer digital 
> equalizers but all of the lines I was involved with under Bell Operating 
> Companies always made used of the 125 Ohms toward the CO and from the CO.
>  
> Dave Hultsman
> Broadcast Channel Manager
> DRS - Continental Electronics .
> Birmingham, AL
> Tel Toll Free (888) 822-1078
> Fax (205) 822-6441
>  
>  
> -----Original Message-----
> From: RichardBJohnson at comcast.net
> To: broadcast at radiolists.net; broadcast at radiolists.net
> Cc: K7qa at aol.com
> Sent: Tue, 10 Apr 2007 8:02 AM
> Subject: Re: [BC] Xmitters at aol.com
> 
> 
> 
> The impedance of various telephone cables is well documented. 16-gauge 
> spiral-four toll entrance cable is 540 ohms -j460, 22 AWG emergency cable is 464 
> ohms -j449, paper insulated (inter-city toll cable) is 910 ohms for 26 AWG, 22 
> AWG is 756 ohms, 19 AWG is 453 ohms, 16 AWG  is 320. All from Page 823, Wire 
> Transmission, Reference Data for Radio Engineers, fourth edition. Nothing is 
> close to 150 ohms. It is well known that the series 500 "standard black 
> telephone" was called the "500" because its hybrid (anti sidetone network) was 
> designed
> for 500 ohms. Historically, the telephone lines have been called "500 ohm" 
> circuits.
> 
> Also, there are no 300-ohm windings on a 111-C. The transformer consists of four 
> identical windings that are electrostatically shielded so they can be connected 
> in any manner. The usual (read correct) connection is for two windings to be 
> series-connected for the output, and two windings to be series-connected for the 
> output.
> 
> See http://www.oldradio.com/current/111c.html , It shows the two usual methods 
> of connection. Note that both the line-side and the drop side can be connected 
> either in series or parallel. Don't be fooled by the "typical use" note. It's 
> for "short" i.e., unequalized circuits.
> 
> I would guess that your response was either a test to see if I was still awake, 
> or not very well thought out!
> 
> --
> Cheers,
> Richard B. Johnson
> Read about my book
> http://www.AbominableFirebug.com
> 
> 
>  -------------- Original message ----------------------
> From: K7qa at aol.com
> >  
> > In a message dated 4/9/2007 6:41:47 AM Pacific Daylight Time,  
> > RichardBJohnson at comcast.net writes:
> > 
> > The  circuit feeding the telco pair should not be connected as 150 ohms  
> > unless
> > there has been a problem obtaining the required frequency response.  
> > 
> > 
> > Most standard balanced telco multiconductor trunk lines used for many years  
> > had an internal surge impedance of 135 to 150 ohms at audio frequencies. The  
> > 111C had individual 300 ohm windings for a good reason.  Connect the  111C 
> > transformer split windings in parallel to achieve 150 ohms on the line side  
> and 
> > in series for the 600 ohm equipment side. 
> >  
> > tm
> > 
> > 
> > 
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