MTR2000 compander

[Bill_G]

MTR2000 – great base station and repeater. You can’t beat it. However, you can make it work terrible by turning on the compander. Drill into the RSS. Go to channel, then to audio. Down towards the bottom you’ll see check boxes for pre-emphasis, de-emphasis, noise canceller, and compander. The Help box has a neat little discussion about how compander is helpful in reducing noise in a system. Anyone who has ever checked the box and listened to the resulting transmit and receive audio simplex can’t really tell the difference. It’s like a do nothing button.

Not true.

The button does something, and it can make a mess of your repeater on a voting system very quickly if every receiver does not employ companding. Any receiver that does not have a compander will have less dynamic range then the receivers with companding, the levels to your comparator will be constantly changing, the resulting voted xmit audio will jump up and down, and the voter will select sites it shouldn’t because they are apparently quieter, not because they have a better signal.

The MTR2000 compander turned on will give you 6db increased dynamic range at the lower end of its receive audio. Let’s say you have the station set up for 0db out with full deviation (+/-4K dev). With compandering turned on or off and full dev injected you’ll see 0db on the rx line out of the station with status tone 13db down. Now reduce dev to +/-3k. With compandering on or off, you see a -3.2db out. Turn dev down to +/- 2k. With compander off you get -6.5db. With compander on you get -7db. Now drop your dev to +/- 1k. This is where you’ll see the big difference – compander off -12db, compander on -18db, a 6db difference. So, at mid to loud deviation the compander does nothing, but at low levels it “expands” the depths of the low signal making it even lower.

It’s assuming the low audio signal is noise, and it’s making the noise smaller so you hear less and thus think the channel is clearer. Unfortunately, many customers have bad microphone technique and barely deviate a radio more than 1 to 2k meaning the compander will make their already quiet voice quieter. And then they will suddenly be much louder as they get closer to the mic – from almost nothing to BOOM I AM IN THE ROOM. And when the voter judges the quality of the many different sites, it will pick the quieter line because that is the better signal even if it is choppy and cuts out and is really really really hard to understand. It is quieter and will get voted.

Turn compander off and leave it off. If you ever wondered what the button does, it generates service calls – that’s what it does. Unless you have a full system of compander portables on a compandered repeater with a compandered console and all the automagic stuff is properly dialed in, leave compandering off in the MTR.

[wavetar]

Yep...it should only be used in a narrowband system, and everything else in the fleet needs to have their various compander settings 'on' for things to work properly. Companding is great when properly used...AT&T actually coined the term, and used a form of companding in their early 90's cordless phones...5200/5400/5500 series phones on 46/49MHz which had phenomenal sound & range comparable to many of today's spread spectrum phones.

[Bill_G]

Agreed. Compandering done well sounds fine. T1 uLaw at the ds0 level has been implemented for forever, and you cannot tell that the processing has occurred. Voice quality is great. However, compandering done poorly, ala Hear Clear, drives people nuts. The MTR implementation of compander is good. You cannot tell by listening that is activated. You can measure it, you can see it's effect on voters that depend on the noise, but you cannot hear the processing. Very smooth. OTOH, compander in an HT1250 sounds terrible. The pumping action is quite audible and mushy. Very distracting to listen to. Perhaps if Motorola implemented the MTR method in all their products it would win me over. For now I turn it off whenever I can.