At 04:44 AM 03/25/08, you wrote: >Vern, > >I have not used a sound card for DTMF decoding, but the decoder on >the Echolink card (I am assuming for the repeaters you are using >one) I find DTMF decoder problems are almost always level related.
Not always >Most decoders work over a reasonably wide range including twist (the >difference between the high and low group tone levels). The ones in >the phone company are usually tighter than most...they are very good. But they are designed to work in systems where the low frequency group tones and the high frequency group tones are close to the same level. There is a chart of the DTMF tones on the web page I referred to above. >However, in radio systems one often does have the advantage of good >flat audio. That's the zinger. It's not. Background info: Each touchtone digit is made up of a high frequency group tone and a low frequency group tone, as explained on <http://www.repeater-builder.com/tech-info/flataudio.html> What many folks, especially newbies, don't understand is that the user's transmitter pre-emphasizes the audio (including all DTMF) 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), for example with a service monitor, it would sound very tinny. If you were to inject 1,000 Hz tone into the microphone jack at a level to get 1khz deviation and then change to 2,000 Hz the deviation would go up 6 dB. 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 jack. This has caught many folks over the years. Many "how-to" writeups over the years have commented that you want to tap into a receiver audio chain to get ahead of the volume control with the purpose of getting a constant level audio to feed, for example, a tape recorder or a repeater controller. If you are not intimately familiar with the recveiver you can tap the wrong point (and sometimes there is no "right point"). Many receiver designs have the volume control ahead of the squelch mute and the de-emphasis. As a result the audio fed to the decoder is NOT de-emphasized "normal" audio and won't decode properly. And it won't be until it is run through a de-emph network. Here's what I suspect you have. I'll bet that you have the DTMF decoder fed with twisted audio and of the wrong level.. >When you say some digits work and some do not this is usually a >level problem and usually requires a separate adjustment from other audios. The first thing to check is the pickoff point - is it truly de-emphasized? If the audio going into the decoder is pre-emphasized, then the high frequency group will be much louder, causing false or no decoding. This can be checked by sending the high tone separately (pressing A and D simultaneously (on a 16-button pad, or 3 and # if a 12-button is all you have) and using an AC voltmeter on the DTMF decoder input, then sending only the low tone (1 and 3 simultaneously) and measuring that audio level. They should be very close. If not, that's your problem. And the audio may be messed up somewhere in the middle. I've seen a situation where the receiver was de-emphasizing properly, but further upstream was a link transmitter with a off-value capacitor and it was over-pre-emphasizing (more than 6db). Any time you have folks messing with random capacitor and resistor values "to make the audio sound right" then things are suspect. You have to measure, and calculate, not guess. By the way, there are audio spectrum analyzer programs that let you "see" the sound card audio. If you have one you can view the twist, which is most visible on the "A" set of DTMF tones. >If the decoder is receiving say over 10% distortion the low group >tones will get into the high group causing problems. As is covered on that web page, and it happens on a lot less than 10% distortion. I quote: >The tone frequencies used in DTMF were well thought out, but not far >enough. Any distortion between the source and the decoder will result >in intermodulation products being generated and the result will cause >unreliable decoding. For example, use the classic 2A-B intermodulation >calculation: The DTMF star button is 941 and 1209 Hertz. Twice 1209 = >2418, minus 941 = 1477. The DTMF pound button is 941 and 1477. So any >distortion in a transmission path can result in a "star" being decoded as >a "pound". This is one example why system designers have to keep >transmission paths linear and distortion free. Otherwise the system >builders will blame the repeater controller manufacturer. And when the >controller goes back to the manufacturer for "repair" he finds nothing >wrong. Becasue there isn't anything wrong. >It may be difficult to check the levels on a sound card...at least >once it gets inside and then to the card audio. > >73, ron, n9ee/r Mike WA6ILQ
