Some hopefully helpful Audio Considerations

Audio connections and other considerations continued

• On the previous page we have discussed the use of a CTCSS also referred to as PL tone decoder and have come to understand that it may be impossible to operate effectively a Echo-Link Node in a busy area where many Nodes operate on the same frequency. Even with such measure as PL tone enabled systems, it is inadvertent that there may still be interference to each other stations. This is something we will have to live with until the Digital RF age has arrived.

This chapter will talk about audio processing / routing in a single transceiver setup

• With our Transceiver from the previous page we will have to use the external speaker as our audio source. There is one word of caution when using the speaker plug. In general the internal speaker gets switch off by some mechanism. This means that the nominal 8 Ohm load will disappear. In order to provide some impedance match a serial resistor of 10 Ohm is suggested to use in the audio path. The real impedance is in most cases unknown and depend on the switching mechanism on the radio. The other concern is that the speaker output can produce power levels far beyond the safe levels your computer soundboard can handle and also depend on the radio circuitry. It is advisable to approach any first time test with low levels. I have heard from others that they fried their Soundboard because the levels went far too high at initial setup. So, start conservative and work your way up. Over driven audio circuits don't produce better sounding audio, they will produce the opposite.
• Too much amplification raises the noise floor and thereby adds un-necessary information to the digitizer, which then in return may reach its capacity and the audio sounds really bad on the distant side. It is better to keep the gain down, just so that the audio is clean or above the noise. In this way the digitizer is working with audio and not noise.
• Most low cost Sound Boards simply don't process the Audio that is fed into them on the analog side in a way as required for this application. In a simplex links with only one transceiver this is not much of a problem. The audio that comes from the speaker circuit is already de-emphasized by the radios receiver circuit. If left alone the audio should sound as received. Echo-Link does a 8 kHz sampling rate with a certain audio codec that I don't want to get into at this time. For my personal experience, a simple setup is sufficient for voice communication. For those that want to be better sounding, there are some ways to process the audio on the analog side before it gets compressed and packetized. To process the audio it is probably best to find an interface that has a circuit built in that allows you to manipulate the audio to a certain degree.
• Transmitting from the Computer to the Radios Microphone input. Most Microphone input circuits are specified to have a 600 Ohm impedance. In general the impedance mismatch is not of too much concern for our application. The levels fed into the microphone amplifier circuit may be more of a concern and is somewhat related to mic impedance. Most mic inputs are perfectly happy with levels around 40 mVpp. Since most of us can't measure these levels, it is primarily an error and trial approach. The best way to verify your audio is to connect to the Echo-Link Test Server and listen to the play back. Keep in mind, if your test transmissions sound good and someone else may complain about bad audio from your system, it may be as well the radio from some other user.
• Ground loops!
• It is very easy to induce ground loops into your system. A ground loop is something that raises your nominal 0 Volt reference to some higher level. If this is the case near audio circuitry, you will get hum and other unwanted sounds onto the path. As little as a few millivolts potential difference between grounds can cause a tremendous amount of hum. In such case it is very time consuming to find the problems and provide good grounds by other means. The objective is to tie all grounds to a common point with the least amount of resistance and inductance.
• There are interface controller that isolate the computer from the Radio. This is primarily to prevent computer generated noise from entering the audio circuitry on the controller board. Those isolation circuits can be quite extensive and usually isolate the Digital grounds from the Analog grounds.

• Using the accessory connector on newer transceiver

• On most newer modern transceiver, you'll find a 6 PIN Mini DIN accessory receptacle on the back or elsewhere on the transceiver. Those connector have usually the same signals as Speaker, Microphone, PTT and in many cases a COR signal available, however at different levels regarding audio. Originally they where designed to allow easy connection to a Packet TNC but have been used also for Internet linking. Kenwood has a Echo-Link ready model in their product line, that does not require anything else aside a Computer with Echo-Link Software. Personally I don't recommend the use of the Accessory connector however it is tempting since it is a very quick way to connect.
• Why am I opposed to this connection?
• First of all the audio circuitry, while in packet mode, on those transceiver is tailored for packet operation. There are in many cases filters that shape the audio to fit the TNC circuitry. Secondly, the Audio levels are in the order of 500 mVpp - 2 Vpp on the Microphone path, lower (~40 mVpp) when using 1200 Baud settings.
•  

PIN	LABEL	NOTE
1	PKD (Data in)	Packet Data input (10 KOhm Imp.) Max. input 40 mVpp (1200 Baud mode) Max. input 2.0 Vpp (9600 Baud mode) Interface may not produce this level.
2	GND	Signal Ground (This maybe an isolated ground)
3	PTT	Most cases is grounded to transmit
4	RX9600	9600 bps Packet Data Output. (Audio from here is filtered differently) The level is commonly at 500 mVpp fixed.
5	RX1200	1200 bps Packet Data Output (Again the audio is filtered/processed differently) The level is commonly at 300 mVpp fixed.
6	PKS (SQL or COS)	Squelch Control, Active at +5VDC, Inactive at 0VDC

Fig.1 Above table shows a very common specification.

 

• The receive audio level, although fixed, is around 500 mVpp into a 10K load impedance at 9600 Baud setting. The receive audio level will drop to ~300 mVpp while operating 1200 Baud. The above is only true for those rigs that allow switching the Baudrate.
• What does this mean to the system setup. If a Interface controller is used that has a DTMF receiver built in, it most likely will fail or operates marginal. This goes true for Software decoder too. The DTMF receiver expects 0dBm signal levels. I have seen them work reliably at 450-500 mVpp levels which is below the 0 dBm. 0 dBm is approximately 770mVpp.  With an input level of 300mVpp it becomes apparent that there are issues. Certainly, one can increase the gain on the Soundboard level, in my experience almost to maximum, unfortunately many cheap soundboards will distort at those levels, Aside the low level problem, the circuitry was tailored for packet, which is in this case 1200 Baud, although not exact but similar to 1200 Hz. This means that the filter may not allow higher frequency components as found in female voices. The male voice will probably sound muffled. Here you have it! It may have been easier and quicker to connect, but you'll sound not optimal and your DTMF commands will work marginal at the best.
• Still, one can use the accessory signal for linking, however with degraded performance.  In many cases an external audio amplifier is needed to get the higher levels need at the interface. To get the best out of it, the radio needs to be modified and signals need to be picked off at different points.  Is it worth it?
• The above mentioned Kenwood transceiver works somewhat different than most all of the other accessory connector. Invisible to most of the user, Kenwood does route the audio differently than it would, using a packet TNC. This is accomplished by bringing the Transceiver into the Echo-Link Mode. Internal circuitry is being switched by firmware appropriately and the Transceiver sounds like most other radios using the Speaker and Microphone connection. Also, Kenwood does not do any DTMF decoding or runs any interface protocol, it rather uses the RTS and DCD signals to communicate statically with the Echo-Link Software. DTMF signals are decoded by Software through audio. My personal opinion is that the Transceiver may provide salvation to an occasional Echo-Link user, but is inappropriate and too expensive for a 24/7 link.

Soon to be added pages that discuss some commonly seen problems with Multi port or repeater linked systems.

Please direct questions to author, thank you!