Error Correction Coding for the WA4DSY 56KB RF modem

I'm offering a free copy of an experimental Error Correction Coded (ECC) EPROM for the WA4DSY / PacComm 56KB RF modem. This may benefit users who suffer from 60Hz line noise, radar pulse noise or just plain weak signals.

Click here to view a block diagram of the encoder.

What it does

Data errors caused by impulse noise will be corrected before being sent to the user interface. This should greatly increase the reliability on links with impulse noise such as radar and 60HZ line noise. Noise pulses 40 db stronger than the signal will not cause errors if they are of short duration. Pulse rates up to 800 pps can be tolerated.

Weak signal performance is improved about 3 DB. For example, I set up two modems connected to each other through 1 db step attenuators. I turned on ECC mode and reduced the signal level until my ping test of 100 packets of 1300 bytes showed a 1% loss. I then switched to normal 56K mode. No packets were properly decoded. I took out 3 db of attenuation to achieve a 3% packet loss. See the scope photos below.

I had good results running ECC on the air through the 56KB full duplex bit regenerator. During cold dry weather defective hardware on a power pole near the repeater site causes considerable line noise. During these periods a user 30 miles distant could only get about 20% of his packets through without ECC. When ECC was switched on the success rate jumped to better than 85%.

Side Effects

Removed features

There are limited resources in the Xilinx FPGA chip. I had to remove some little used functions to make room for the ECC functions. The ECC version can't respond to a remote control signal and it can't be used as a stand-alone bit repeater. However, it can still send a remote control signal and the ECC signal will pass through a standard non-ECC 56KB bit repeater.

Speed

The ECC uses a rate 2/3 code. This means the users data rate is 2/3 the modems bit rate because there is one parity bit transmitted for every 2 data bits. The maximum through-put is 37.3KB.

The DCD (Data Carrier Detect) response time is much slower than the non-ECC version. It can take as long as 8 MS from TX keyed to valid data. I suggest setting your TXDELAY parameter to 16 ms or more if you are going through a full duplex bit repeater.

The DCD signal will go true before valid data is available at the user interface. This is the opposite of a squelch tail which has always existed in WA4DSY modems. This "squelch header" hasn't caused any problems for my Ottawa PI-2 cards.

Clock Signals

The modems receiver clock output is not a square wave. It looks like a 56khz clock with every 3rd pulse missing. In fact, this is exactly how it is generated. This is a side effect of the 2/3 bit rate reduction and the suppresson of the parity bits on the user interface. If anyone has a problem with this please drop me an email. Valid data is always present on the rising edge.

Compatibility

Modems with the original EPROM can't communicate with the ECC version. Original 56KB and ECC modems can coexist on the same frequency because the carrier detect (DCD) works the same in both models. Since they recognize the presence each others signal, collisions can be avoided even if data can't be exchanged. The frequency memories in this EPROM are the same as the production 56KROM04.BIN which is currently shipped with the modem. 
Memory   TX Freq   RX Freq   RC Code   Tuneup Freq
 ______   _______   _______   _______   ___________
   0      29.050    29.050      20       29.050
   1      29.950    29.950      20       29.950
   2      28.400    29.850      20       29.850
   3      29.850    28.400      20       28.400
   4      28.550    29.550      20       29.550
   5      29.550    28.550      20       28.550
   6      28.000    28.000      20       28.000
   7      30.000    30.000      20       30.000

Mode Selection

The ECC version can operate in both plain 56k and ECC modes by setting DIP switch S4-3 to the desired mode. This switch is labled "repeater ON". Since there isn't a repeater function in the EPROM I used that switch to change modes. When S4-3 = OFF the modem is in normal 56KB mode. When ON it's in ECC mode. I've also connected the mode select to uninstalled front panel switch S5. A DigiKey part number EG1102-ND switch can be installed if desired. AN EG1192-ND cap will also be required. You also need to drill a hole in the front panel.

NOTE: This is an experimental EPROM. This is the first attempt at doing ECC. The next version, if any, may not be backwards compatible.

Downloading

Click here to download 56KEC04A.Zip , the EPROM image. (23K)
Unzip the file and program 56KEC04A.BIN into a 27C256 150ns or faster EPROM and replace the original. Dip switch S4-3 must be ON to enable the ECC mode.
This EPROM code is Copyright 1996 by Dale Alan Heatherington

Results of sending 100 packets of length 1300 bytes with and without ECC

0DB relative signal level


Eye pattern observed for 3% packet loss
without error correction coding

-3DB relative signal level


Eye pattern observed for 1% packet loss
with error correction coding.
This is 3DB less signal than the first.
100% packet loss occurs at this signal
level without error correction coding.