[RTTY] 300hz or 500hz IF filter?

[Kok Chen]

On Aug 25, 2013, at 2:20 PM, Bill Turner wrote:

> Long experience has shown that around 300-350 Hz on the typical ham receiver
> works. Less does not.

To paraphrase the other Bill (Clinton), that depends on what you mean by "works."

If you have built and adjusted demodulators before, you know that they don't suddenly shut down.  They simply degrade.

By how much do they degrade?   Again, I recommend that you look *very, very* carefully (be sure to look at the scales, not just the shape of the curve) at Figure 2.2 here of what cascading an extra filter does to RTTY error rates:

http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html

Notice that the error doubles from an ideal 300 Hz filter to an ideal 210 Hz filter.  

Doubling error is never a good thing (especially if you are providing cell phone service), but look at the scale again.

The ISI from cascading an extra filter (e.g., the filter in your superhet) adds an extra 1 error per 250 characters for the 300 Hz filter, and adding 1 error per 100 characters for an ideal 200 Hz filter.

An extra 1 error per couple of seconds may bother guys like me, but probably won't cause a contester to worry -- most exchanges are fewer than 100 characters long.

I venture to add that a contester can probably tolerate an extra error rate of even 1 error per 50 characters -- in which case, an ideal 180 Hz filter will work.

So, any practical filter that can pass 180 Hz  to 200 Hz worth of clean passband will probably work more than sufficiently for contesting.  If you want to dig out the weak DXpedition, you will need every little bit of reduced error as you can, since SNR is not in your favor to start with, and that is where you will need to widen the bandwidth.

A filter is flat and has less than 1 ms of group delay for 180 Hz is probably good enough for contesting.

Us modem designers are trying to squeeze every drop of blood from our modems (I know that Dave W1HKJ, Stefan DO2SMF and David G3YYD are constantly trying eek out an extra percent or two fewer errors with their modems).  So we absolutely care about 2x type errors (in the end that will benefit everybody.  But that 2x of errors from ISI is not going to bother most contesters.

73
Chen, W7AY