75 uSec Pre-Emphasis Graph

FM does it with frequency!
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jthorusen
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75 uSec Pre-Emphasis Graph

Post by jthorusen » Wed Jun 26, 2019 10:42 pm

Does anyone have a clear, readable graph of the U.S. 75 microsecond pre-emphasis curve? A pdf or a high res jpg or png file would be dandy. The only one I can find online is only about four inches squared. I'd like one that takes up a whole sheet of 8 1/2 by 11 paper. We used to have at least a ream of printed graphs at the KCBS transmitter, but I don't have a clue what happened to it after the building was sold. Probably got circular filed.

Anyway, a printable copy would be very much appreciated.

Thanks,
James K. (Jim) Thorusen
KB6GM
Central Coast Electronics
www.centcoast.com
NW Oregon Consulting Bdcst Eng.

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Re: 75 uSec Pre-Emphasis Graph

Post by kcbooboo » Thu Jun 27, 2019 5:50 am

It's not the best, but there is one on this web page, plus there's a link to a PDF version of the article:
https://www.ic.gc.ca/eic/site/smt-gst.n ... 01237.html

The standard 75 uSec pre-emphasis provides a 6dB increase per octave above approximately 400 Hz. It could be as simple as a 1uF cap in series with the audio followed by a 75 ohm resistor to ground; anything that forms an RC time constant of 75 uSec.

I would think that an Excel spread sheet can be written to perform the math since Vout = Vin * (R / (Xc + R)) then plot the results on logarithmic X and Y axes for frequency and output level.

Bob M.
Last edited by kcbooboo on Thu Jun 27, 2019 6:52 am, edited 1 time in total.

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Bill DeFelice
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Re: 75 uSec Pre-Emphasis Graph

Post by Bill DeFelice » Thu Jun 27, 2019 6:31 am

I remember seeing it in an old Gates manual for one of their exciters. If I happen across it I'll do a scan and post.
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Re: 75 uSec Pre-Emphasis Graph

Post by kcbooboo » Thu Jun 27, 2019 6:53 am

Lots of those old FM TX manuals had that graph. I presume even the FCC's regulations had it at one point. It might even be in some older CBS FM Volumax manuals.

Bob M.

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Dale H. Cook
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Re: 75 uSec Pre-Emphasis Graph

Post by Dale H. Cook » Thu Jun 27, 2019 8:58 am

Years ago I scanned the NAB "Instructions for Conducting Audio Frequency Proof-of-Perfomance Measurements" instructions and forms for a fellow engineer. I have pulled the pre-emphasis graph and posted it at https://plymouthcolony.net/starcityeng/ ... AB-Pre.pdf . It is an old scan done with an old scanner. If the size is okay but you need a better scan it may take a few days to locate the original and get a better scan.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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Re: 75 uSec Pre-Emphasis Graph

Post by kkiddkkidd » Thu Jun 27, 2019 12:09 pm

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Re: 75 uSec Pre-Emphasis Graph

Post by DaveSt » Mon Jul 01, 2019 9:23 am

There are some very simple browser based circuit simulators. For example https://www.circuitlab.com/. For pre-emphasis you can do a parallel RC combination into the non inverting input of an op-amp. If you have the circuit of the pre-emphasis part of whatever box you have, you can simulate it exactly, including the shelving resistor. You get plots for amplitude and phase.

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jthorusen
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Re: 75 uSec Pre-Emphasis Graph

Post by jthorusen » Wed Jul 10, 2019 1:57 am

Greetings and thanks to all who responded. For some reason, I do not get E-mail notifications of replies to my posts here and I often for get to check back in a timely fashion. Please accept my apologies.

For kkiddkkidd: That's the graph I found and complained about its being too small. However, when I printed it out, I managed to get it to pretty much fill the page and it is even clear enough to read! Thanks!

For Dale Cook: Being too poor to buy an NAB engineering handbook, I'd love to get a copy of your complete proof document. Unfortunately, the link you posted to the pre-emphasis graph does not work.

Thanks to everyone else.... If I can get a copy of dale's proof docs, I will be all set.

vy73,
James K. (Jim) Thorusen
KB6GM
Central Coast Electronics
www.centcoast.com
NW Oregon Consulting Bdcst Eng.

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Dale H. Cook
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Re: 75 uSec Pre-Emphasis Graph

Post by Dale H. Cook » Wed Jul 10, 2019 7:03 am

jthorusen wrote:
Wed Jul 10, 2019 1:57 am
Being too poor to buy an NAB engineering handbook, I'd love to get a copy of your complete proof document.
It is not from the NAB Engineering Handbook. It is an entirely different NAB publication, "Instructions for Conducting Audio Frequency Proof-of-Performance Measurements" published in 1877. I have posted the full proof instructions at:

https://plymouthcolony.net/starcityeng/ ... _Proof.pdf

and the full set of proof forms, the last of which is the pre-emphasis graph, at:

https://plymouthcolony.net/starcityeng/ ... _Forms.pdf
jthorusen wrote:
Wed Jul 10, 2019 1:57 am
Unfortunately, the link you posted to the pre-emphasis graph does not work.
One incorrect character in a link is all it takes - that has been fixed.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html

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Re: 75 uSec Pre-Emphasis Graph

Post by kkiddkkidd » Wed Jul 10, 2019 3:19 pm

Dale H. Cook wrote:
Wed Jul 10, 2019 7:03 am

Audio Frequency Proof-of-Performance Measurements" published in 1877. I have posted the full proof instructions at:

And they used candles to proof read it... Edison took 2 more years to invent the light bulb.
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jthorusen
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Re: 75 uSec Pre-Emphasis Graph

Post by jthorusen » Wed Jul 10, 2019 6:44 pm

Thanks, Dale!

Both sets of documents saved to my hard drive.

A quick perusal of the FM stereo instructions states that a stereo modulation monitor is required. When I first got involved with KWPB, they had no modulation monitor of any sort. I found an antique TFT 723 on eBay and bought it. After a monumental struggle lasting over a year, I finally got it moved in frequency and operating.

(I created a Word document detailing all the changes and fixes that I had to do and it runs to seven pages in outline form... if anyone is interested, I can post it. Darryl, who is doing TFT support on this Forum, already has copies of everything I have.)

So now, we can keep an eye on our total modulation and our carrier frequency. However, I haven't found a stereo monitor within my budget (I am buying this stuff out of pocket since the station is under-funded). So, I'm not sure what sort of a proof I can really do.... but any measurements I can make and place in the files should be better than nothing if we ever get inspected.

Thanks again,
James K. (Jim) Thorusen
KB6GM
Central Coast Electronics
www.centcoast.com
NW Oregon Consulting Bdcst Eng.

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Re: 75 uSec Pre-Emphasis Graph

Post by W2XJ » Tue Jul 23, 2019 4:25 am

You can do anything necessary with almost any scope. Regarding 75us pre-emphasis, I haven’t used that for over 30 years. 50 us or less is a much better option.

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