rjr@mgweed.UUCP (Bob Roehrig) (05/22/85)
People that manufacture FM gear for commercial uses (at least the BIG manufacturers like Motorola and G.E.) paid attention to things like proper pre-emphasis and de-emphasis curves in their transmitters & receivers. That is why the audio sounds better. Also they tailored the response by careful selection of the matching speakers. What is desired is not HI-FI but most effective communications audio. Several years ago, I reworked my ICOM 22-S because the audio was so "thin" that I could not recognize who was talking to me. With no modesty whatsoever, I must say that I paid special attention to the 2 meter repeater that I built and that its frequency response is flat between 300 and 3000 Hz. Also, the transmitter deviation is exactly what the received deviation is. The main problem with the ICOM was that the low end was rolled off. This was probably done because the amp is low power (less than 1 watt, as I recall) and if it had full range response, it would have distorted at higher levels needed in a noisy car. During my travels around the country, I hear some pretty lousy sounding repeaters! I think they must have just connected the transmitter input directly to the rcvr speaker output, without regard for response. In receivers with transformer outputs, such as a Motorola Motrac/Motran or older tube types like the Sensicon "A" etc, the output at the speaker terminals does not follow the standard de-emphasis curve. You have to get back a stage and add a follower to get the proper response for repeater use. Another problem FM'ers face today is the way most "jap-track" rigs are set up on transmit. Most have way too much mike gain. The deviation may be set properly but usually you can hear a fly fart in the next room! I am told that these rigs are set up this way because the oriental voice/language is not as "harsh" as ours, which requires a more sensitive speech amp setting. At least some of these rigs have a mic gain pot as well as a deviation control. When the gain is too high, there is excessive clipping, which makes it harder to understand the person. Bob K9EUI