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FM Audio Understanding (Pre-emphasis, De-emphasis and Flat)

Discussion in 'General Technical Questions and Answers' started by K8ATG, Jul 11, 2019.

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  1. K8ATG

    K8ATG Ham Member QRZ Page

    I’m looking for some help on checking if my understanding of FM audio (de-emphasized, pre-emphasized and flat) is correct. I’ve had some problems on figuring out why two repeaters that are linked together have poor audio when a field user is on one repeater and another field user is on the different repeater. I’ve sketched out an audio block diagram to try and visualize how the audio characteristics are being passed through the system.


    To highlight, the repeater system consists of two repeaters where one is VHF and the second is UHF. They are linked by RF with an extra transceiver on each controller port, that passes audio on a 220 MHz frequency from site to site.


    My understanding is that a user radio’s will have a pre-emphasis circuit when transmitting and a de-emphasis circuit when receiving (referring to normal HAM grade transceivers such as Yaesu, Alinco, Icom, Kenwood, etc). This is what happens when two users are transmitting by simplex. User A transmits, his audio is then pre-emphasized and transmitted, user B receives his transmission where his radio de-emphasizes the audio and is presented to user B.


    With this understanding, I’m assuming the correct thing to do is to keep audio passed through the repeater controller and link transceivers completely flat. This means that the audio being presented on the receiver has the exact same characteristics as when it is passed to the repeater transmitter. The interface or link transceiver will not shape or “touch” the audio as its being passed. If this was the case, I see where it could be possible to de-emphasized or pre-emphasize the audio twice.


    If my understanding is correct, each repeater and the controller should be configured to pass “flat” audio and basically allow the user in the field to handle the pre-emphasis and de-emphasis.


    Could someone tell me if my understanding is correct and if how I have sketched my audio block is correct?
     

    Attached Files:

  2. K7JEM

    K7JEM Ham Member QRZ Page

    Your diagram looks correct, as far as a cursory examination. The way pre and de emphasis is handled in a repeater is largely due to how a particular manufacturer does it, or how a particular controller is set up. Because it is easy to confuse terms, I like to use the terms "discriminator audio" and "modulator audio" when referring to "flat" audio. I then call the other points "mic audio" and "speaker audio" when referring to the "non emphasised" audio. This would be referred this way, regardless of where it appears, IOW, as long as the audio has not been reshaped in some significant way.

    Most commercial radios feed audio into the TX at a "microphone" point. Many ham grade repeaters maintain disc/mod (flat) audio throughout the whole chain. This requires all audio introduced to be of the same type. You can't take modulator audio and feed it into a mic input, you will get "double emphasised" audio. Likewise, you cannot take speaker audio and feed it into a disc point, you will end up with "double de-emphasised" audio. Either of these will sound bad, the first instance will sound "tinny" and the second will sound "muddy" or overly "bassy".

    All you have to do is keep all of the inputs and outputs the same, either by attaching directly to the proper radio points, or "converting" the audio externally from one format to the other. This can be as simple as a resistor/capacitor network, but sometimes will need an op-amp or some other type of amplification.
     
    K8ATG likes this.
  3. AI3V

    AI3V Ham Member QRZ Page

    Is it poor audio, or different audio?

    While your understanding is correct in a general sense, in practice its rather difficult to switch audio paths without a noticeable change in the sound due to slight differences in the frequency response of the various parts.

    And the more links in the chain, the harder it gets.

    Its standard practice in a multi receiver "voting" system to use identical makes and models of receivers.

    This is one big advantage of a all digital system, no matter how many links in the chain the signal is not degraded.

    Rege
     
  4. AA5CT

    AA5CT Ham Member QRZ Page

    You can't route analog over telco, they won't lease you the lines ...
     
  5. AA5CT

    AA5CT Ham Member QRZ Page

    re: "My understanding is that a user radio’s will have a pre-emphasis circuit when transmitting and a de-emphasis circuit when receiving "

    per EIA/TIA standard; it improves the SNR, intelligibility.
     
  6. K7JEM

    K7JEM Ham Member QRZ Page

    You can lease an analog line, but it might be converted to digital somewhere along the way. Especially if it goes a long distance. Then converted back.
     
  7. AI3V

    AI3V Ham Member QRZ Page

    What does that have to do with the price of bananas in Equidor? :)

    Rege
     
  8. K8ATG

    K8ATG Ham Member QRZ Page

    The audio is poor but only when going through the link and listening on the opposite repeater. The audio of the repeater sounds perfect is each user is on the same repeater. Once two users are on different repeaters, the audio quality is much much less.

    A little off topic but these responses lead me into my next question or area that I'm trying to understand...

    Is anyone familiar with Motorola CDM series and how the audio pins and options in the software is handled? The links between the repeaters are handled by CDM's. For instance, I would believe I DON'T want to use pre-emphasis and de-emphaiss options in the software and that the audio output pin should be configured as flat audio. Transmit audio should be connected on pin 2 (mic audio). This should allow that no audio shaping is done between the radios and acts just like the original repeater receiver and transmitter.

    CDM
    Pin 11 - RX Audio Out (Configured as flat audio out in CPS)
    Pin 2 - External Mic Audio In (Configured as NO pre-emphasis or de-emphasis)

    These settings above should pass audio exactly how it was received. No changes to the audio deviation would be made. Am I correct in thinking this?
     
  9. KF5LJW

    KF5LJW Ham Member QRZ Page

    No you cannot lease an Analog Line at least in the sense I understand you speak of. Ma-Bell and the industry quit using analog long ago and there are no more 2 or 4-wire leased analog circuits anymore. It may be analog on the copper pair between you and the Central Office, but at the Central Office is converted to digital DS0 56/64 Kbs immediately to be muxed into a DS1 for transport for either T-carrier or Ethernet. All radio and television uses digital from studio to transmitter. Analog is dead in telecom.
     
  10. KF5LJW

    KF5LJW Ham Member QRZ Page

    Your understanding is correct, just foggy in the middle with respect to the Repeater. The Repeater can be either Flat or Emphasized which is transparent to the end user.

    Example say Motorola Micor or GE Executive conventional FM Repeaters use the exact same radios as the mobile version. Only difference is the TX PA in Repeaters was 100% duty cycle where the mobile is only 20%. The RX de-emphasized, and the TX emphasized, thus making it flat and transparent to the user because what you receive is emphasized.
     

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