An Explanation of “Flat Audio”
By Jeff DePolo WN3A
Flat Audio:
When most of us talk about “flat audio”, we’re talking about passing audio through the repeater without any de-emphasis/pre-emphasis stages in line. Discriminator audio that has already been pre-emphasized in the user’s radio gets passed to an FM (not PM) modulator without any intervening de-emphasis in the receiver or pre-emphasis in the exciter. Of course there should still be limiting and low-pass filtering. Most controllers can be adapted to handle “flat audio” (i.e. pre-emphasized audio from the discriminator). This usually involves adding some de-emphasis on the audio going into the DTMF decoder and the send side of the autopatch interface, and some pre-emphasis on the speech, tones, and receive side of the autopatch interface.
So in reality, “flat audio” is somewhat of a misnomer, since the audio you’re dealing with through the repeater isn’t flat at all, it has been pre-emphasized in the user’s radio. My guess is the term caught on because people were bypassing the stock audio circuitry (de-emphasis/filtering in the rx and pre-emphasis in the tx) to make the overall audio response “flatter” in terms of minimizing the low-end and high-end rolloff.
Flat audio is one of those phrases that can be used two different ways that are polar opposites of each other. In narrowband FM like we use on amateur, and commercial repeaters, the user’s transmitter pre emphasizes the audio at a rate of 6 dB per octave. In laymen’s terms, the higher frequencies are transmitted at a higher deviation than the lower frequencies. If you were to listen to “raw” FM audio that has been pre-emphasized, it would sound tinny.
In a normal repeater, the audio that was pre-emphasized in the user’s radio gets de-emphasized in the repeater receiver, thus returning it back to normal status. The audio then goes through the controller or other audio stages in the repeater control shelf. When it gets to the repeater transmitter, it gets pre-emphasized again. On the receiving end, the user’s radio de-emphasizes the audio that was pre-emphasized by the repeater transmitter, returning it to normal audio on par with what went into the originating station’s microphone.
However, the de-emphasis in the repeater receiver and the pre-emphasis in the repeater transmitter can be eliminated together as a pair since they are reciprocal. What gets received by the repeater receiver as pre-emphasized audio can go back out the repeater transmitter with the original pre-emphasis kept intact. This is what is often referred to when people talk about “flat audio mods” in a repeater installation. In reality, whether there is a de-emphasis/pre-emphasis pair done in the repeater, or whether the repeater skips the de-emphasis/pre-emphasis steps, a repeater always repeats “flat” audio. It comes in pre-emphasized and it goes back out pre-emphasized either way.
There are many considerations to be made when skipping the de-emphasis/pre-emphasis steps in a repeater, primarily those pertaining to audio sources aside from the repeater receiver (such as the controller’s synthesized speech, the controller’s tone generators, audio to/from the autopatch, etc.). In such cases, these ancillary audio sources need to be pre-emphasized separately, something that most controllers don’t provide provisions for doing, since the transmitter won’t be providing the necessary pre-emphasis on its own.
There are other technical considerations with regard to how to get around the stock audio processing in repeaters as well as the pre-emphasis/de-emphasis stages in the receiver and transmitter. These aspects require particular attention to detail, especially surgery to the transmitter which can easily result in causing interference in the form of adjacent channel splatter, spurs, and broadband noise.
Jeff DePolo WN3A, January 19 1999
An Explanation of “Pre-emphasis, & De-emphasis”
By Kevin K. Custer W3KKC
Emphasis:
The concept of Pre-emphasis and De-emphasis is a broad subject, however I will try to give you the basic concept behind the advantages, and necessities (or lack of them) in NBFM.
In common narrow band two way fm communications, Pre-emphasis follows a 6 dB per octave rate. This means that as the frequency doubles, the amplitude increases 6 dB. This is usually done between 300 and 3000 cycles. Why is it necessary? Pre-emphasis is needed in FM to maintain good signal to noise ratio. Common voice characteristics emit low frequencies higher in amplitude than high frequencies. The limiter circuits that clip the voice to allow protection of over deviation are usually not frequency sensitive, and are fixed in level, so they will clip or limit the lows before the highs. This results in added distortion because of the lows overdriving the limiter. Pre-emphasis is used to shape the voice signals to create a more equal amplitude of lows and highs before their application to the limiter. The result is that the signal received is perceived louder due to more equal clipping or limiting of the signal, but probably more important is the increased level of the higher frequencies being applied to the modulator results in a better transmitted audio signal to noise ratio due to the highs being above the noise as much or more than the lows. So what is the original reason for Pre-emphasis? Bob Schmid covers that in the last section of this page.
Transmitters that employ a true FM modulator require a pre-emphasis circuit before the modulator fore the true FM modulator doesn’t automatically pre emphasize the audio like a transmitter that uses a phase modulator. A separate circuit is not necessary for pre-emphasis in a transmitter that has a phase modulator because the phase modulator applies pre-emphasis to the transmitted audio as a function of the modulator itself. In other words the phase modulator ‘automatically’ pre-emphasizes the applied audio.
The user’s receiver de-emphasis circuitry takes the unnatural sounding pre-emphasized audio and turns it back into its original response. Pre-emphasized audio is however available directly from the audio demodulation or more commonly the ‘discriminator’ circuitry.
In linking systems, many choose to eliminate the emphasis circuitry (on a whole) to allow better representation of retransmitted signals. Since the signal has already been pre-emphasized (by the user that is transmitting,) and since the receiver you are listening to takes care of the de-emphasis…. it doesn’t need to be done over and over again.
Some loss of quality does exist in flat systems, but quality is better maintained by its use. A flat audio response system is one which has equal output deviation for the same input deviation, no matter what the applied audio frequency is…..within reason.
Reasonable audio frequency response would be from 50 cycles to about 3500 to 5000 cycles in a system not filtering the CTCSS tone. Audio response in a system that does filter the CTCSS would be around 250 to about 3500 to 5000 cycles. The upper cut off frequency would be determined mainly on acceptable use of available bandwidth….3500 on a 15 kHz 2 meter pair, or 5000 on a 25 kHz UHF pair.
Available Bandwidth:
Injecting discriminator audio back into an FM modulator without any limiting or low pass filtering is bad news, plain and simple. On UHF, you may be able to get away with it without excessively bothering either of your adjacent channel neighbors, but on 2m, especially with 15 kHz spacing, you’d be asking for a lynching.
Without low pass filtering, all of the high frequency energy (hiss) that comes from the discriminator from a noisy user, if not low pass filtered, will deviate your transmitter in excess of 5 kHz, in addition to pushing the sidebands out further than they would be if the modulating frequencies were cut off at/about 3 kHz. Do this to see what I am trying to get across. Set your repeater up for 1:1 input to output ratio (like, put in a signal that is deviated 3 kHz by a 1 kHz tone, and set your Tx audio gain to get 3 kHz out of the transmitter). Now open your repeater receiver squelch wide open. You should see your transmitter is now deviating somewhere around 8 or 9 kHz (presuming you have enough audio headroom through the controller). Under this test condition, the combination of the excessive deviation and the lack of high frequency filtering will make your signal somewhere around 30 kHz wide instead of 16 kHz as it should be (very high modulation index.) The only thing “limiting” the occupied bandwidth at that point is the dynamic range of the audio circuits in the controller & radio… and the natural high frequency rolloff of the discriminator’s output noise. Observe the occupied bandwidth on a spectrum analyzer to view this.
Obviously that’s a worst case scenario, but the fact remains that you should have brick wall limiting at 5 kHz (a little lower at 15 kHz channel spacing), and possibly low pass filtering at 3 kHz (a little higher is OK on 25 kHz channels).
Summary:
A repeater can be built to utilize a flat audio response to maintain quality through the system. This is fairly easy in a system using a true FM modulator. Usually some modifications to the controller are necessary to allow it, especially ones that have a speech synthesizer or a phone patch. Systems using a phase modulator require de-emphasis before the modulator because of the unavoidable built-in pre-emphasis of the audio by this type of modulator. For this reason… it is easier to utilize flat audio modifications and maintain quality audio in a system employing a true FM modulator in each transmitter.
Kevin Custer W3KKC, January 19 1999
Flat Audio in action on two way radio Repeater
We must recognize that early narrowband radio was intended for one transmitter, one receiver applications. This business of linking repeaters came much later. We pre-emph the audio to the FM transmitter to simulate PM, but must maintain a narrow bandwidth to be a good neighbor. So, we roll off the audio at, let’s say, -3 dB at 3 kHz. If we hop through another similar system, the resulting audio is then down another 3 dB at 3 kHz, or a total of -6 dB at 3 kHz. This narrowing of the audio bandwidth is what everybody complains about in linked systems.
So, the popular answer is to eliminate de-emph and pre-emph in the repeater. Just feed the user’s pre-emph’d audio from the repeater receiver’s discriminator to the repeater transmitter after the pre-emph stage, thus bypassing the repeater’s de-emph and pre-emph circuits, resulting in a “flat” repeater, right?. (Of course, you still have all those controller mods to make.) Everyone then assumes de-emph and pre-emph are evil!!! They must be, since the audio sounds better without them!
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