> -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of nj902 > Sent: Wednesday, March 14, 2007 6:55 PM > To: [email protected] > Subject: [Repeater-Builder] Re: fixed-audio? > > --- In [email protected], [EMAIL PROTECTED] wrote: > "... Our system takes flat audio in and delivers flat audio out..." > > You make my point. You say "Our system" You are a controller guy. > Your controller is your perspective. But that is actually a device > perspective. > > Your device wants "flat" audio in and out. Flat in the sense that > it is neither pre-emphasiszed or de-emphasized as it arrives at your > device and as it leaves your device. Flat in the sense that you > won't alter the frequency response of that audio on its way through > your device. > > The receiver that precedes your controller is another device. From > its perspective, de-emphasized audio is not flat. It can't be by > definition. If you are connected to de-emphasized audio, the > receiver IS changing the frequency response. If you are connected > to discriminator audio, it is "flat", i.e. not changing the > frequency response. > > Keeping a clear perspective on the differences between devices and > systems IS the issue. At any given point in a system, audio may be > flat or it may not be - with respect to some other point. For a > clear understanding of the system, we should also clearly understand > the action of each device. > > Also, with regard to this statement: > > "... can't change the flatness. Pre-emphasis is always precisely > +6.000 dB/octave and de-emphasis is always precisely -6.000 > dB/octave. " > > Real world transmitters always have limiters. Those DO change > flatness. Just look at the EIA/TIA specication for testing > transmitter pre-emphasis. The test is not run at system deviation. > It is not even run at 60% of system deviation. It is run at 20% of > system deviation. [that's +/- 1 KHz deviation for 5 KHz systems] > > Run the test at 20% into a modulation analyzer and you get a nice 6 > dB per octave line from 300 to near 3000 Hz. > > Run it again at higher deviations and see what the limiter does to > your nice straight line - the pre-emphasis curve hits the limiter at > progressiviely lower and lower frequencies as you increase the > deviation. The result of this fact is going to alter the audio > characteristic going through your repeater. It's one reason why > level setting is so important. It is also a significant reason why > it's difficult to get a repeater to sound like simplex. > > Another post suggested checking the frequency response of your > repeater. Definitely - do that. Try it a various deviations. You > may be surprised at how ugly it gets. > >
------------------------------------------- It seems many are making this subject more complicated than it really is. If we call audio at a particular point what it has been called for over the last 50 some years it is much easier to stay out of trouble. Audio that comes directly off the discriminator is called "discriminator audio". Discriminator audio has the pre-emphasized audio that the transmitter transmitted. Audio that comes after the de-emphasis network is called "de-emphasized audio". It has the same audio that went into the microphone of the transmitter, hopefully. The following is for the benefit of the newer guys. Maybe some old guys too. In a repeater the de-emphasized audio from the receiver is usually fed to the audio input of the transmitter where it is again pre-emphasized before it is applied to the transmitter for modulation. Lots of people don't know the difference between audio level and deviation. That gets folks into trouble when trying to set up a radio or repeater properly. Years ago Helper Instruments made a monitor with a deviation meter and another meter they called "modulation density". The modulation density was the measure of average audio level. Running a pre-emphasis curve on a transmitter must always be done below the point of any limiting. If you think about why, it makes perfect sense. The limiter is AFTER the pre-emphasis circuit. The pre-emphasis circuit raises the amplitude of the high frequencies of the audio. The limiter limits the amplitude of ALL frequencies applied to it equally. So naturally the high frequencies are going to be the first ones limited. That's one of the reasons why a transmitter that has the deviation set properly but the audio gain too high sounds mushy. That unbalances the pre-emphasis curve. Higher gain allows the low frequencies to reach near the same deviation level as the high frequencies that are being limited by the limiter. As the gain is increased the low frequencies start to hit the limiter and become the same amplitude as the high frequencies. Good by pre-emphasis! Now when the receiver sees this heavily limited audio its de-emphasis circuit is going to try and equalize what it thinks is pre-emphasized audio to try and make it sound normal like it was when it went into the transmitter. But because the low frequencies are at such a high level (they are near the same level as the high frequencies) when it rolls off the high frequencies as the normal de-emphasis circuit does, it leaves very high level low frequencies compared to the high frequencies. This gives a muddy base sound but loud and of course it also contains the distortion products generated by the heavy limiting. For those that are new to setting up FM radios, you always set the deviation control first, to 5 KHz or close to it. Then adjust the audio gain for proper loudness. Don't adjust the deviation control to try and control loudness. 73 Gary K4FMX
