digital scanner error-correction

[DaveH]

As far as I understand, 2.4k of the 9.6k Astro data stream is allocated to
error-correction. I'm not sure if that includes the embedded signalling as well
as voice but it would make sense.

How much do "consumer-grade" scanners make up of the correction data?
Certainly "real radios" do, but some scanner users have been heard to say that
the signal quality depends on having a strong signal ("being close to the tower").
That would suggest that they don't do correction as well (or do they cut corners
and not do it at all, just drop bad frames?).

Regards, Dave

 

[ianw]

I can't say I know anything specific about the code in the digital scanners
on the market today but I would imagine it depends on the type of error
coding used in APCO25.

If it is some kind of block coding then yes I have seen decoder products
that simply ignore the error correction data or slightly better some which
use this data for a simple yes/no decision on if it should use the data. Of
course best of all are the products that use the block coding to try and
actually correct bad bits.

However if convolution coding is used then the programming really has to
error correct just to decode the data. Mind you there are better ways of
doing since soft decision decoding is supposed (in theory) to provide a
2db improvement over hard decision decoding.

Does anyone have any info on APCO25 error coding ?

Regards

Ian

 

[rdale]

" How much do "consumer-grade" scanners make up of the correction data?"

The Pro-96 doesn't "Max Headroom" like my Motorola W7 does on weak signals... It's much more legible.

- Rob

 

[N_Jay]

" How much do "consumer-grade" scanners make up of the correction data?"

The Pro-96 doesn't "Max Headroom" like my Motorola W7 does on weak signals... It's much more legible.

- Rob

Or the Motorola is trying to decode when the scanner would have already mutted.

 

[ianw]

Remember that Motorola handhelds are more sensitive than a scanner
and have better filtering. Just because a scanner sometimes receives
choppy audio doesn't mean it isn't error correcting.

Also in poor reception areas you will sometimes have times where some
of the data packets received are to badly damaged to be error corrected
and that those times any digital radio will sound strange. When I am in
the basement at work and receive a call on my GSM mobile phone it
can sound very odd indeed.

Ian

 

[rdale]

"Or the Motorola is trying to decode when the scanner would have already mutted."

That may be - the point is that the Pro96 allows at least some form of listenability to transmissions that the Mot makes useless...

 

[N_Jay]

"Or the Motorola is trying to decode when the scanner would have already mutted."

That may be - the point is that the Pro96 allows at least some form of listenability to transmissions that the Mot makes useless...

That was not the point..
I would guess the Mot is trying to provide soem intelegibility at a signal level where the scanner is completly quiet.

 

[rdale]

Okay - let's try this again. You take whatever point you want because you have me confused. When the Motorola "Max Headrooms" the Pro96 just drops the stuttering. You cannot figure out what the person is saying on the Mot, you can on the Pro96.

 

[N_Jay]

Okay - let's try this again. You take whatever point you want because you have me confused. When the Motorola "Max Headrooms" the Pro96 just drops the stuttering. You cannot figure out what the person is saying on the Mot, you can on the Pro96.

We would have to put it on a calibrated test set to prove this out, but my assertion is that at the signal level that the scanner has muted (no audio), the commercial receiver will still be receiving, maybe with some errors adding artifacts to the received signal, but often still inteligible.

 

[rdale]

You may assert or testbed all you want... The end result is that Motorola radios can Max Headroom - the Pro96 doesn't. When it does into Max Headroom mode, the voice is not usable at all. The Pro96 allows you to still make out much of the conversation.

 

[N_Jay]

You may assert or testbed all you want... The end result is that Motorola radios can Max Headroom - the Pro96 doesn't. When it does into Max Headroom mode, the voice is not usable at all. The Pro96 allows you to still make out much of the conversation.

But if you don't know the receive level, you don't know which is working better. Maybe the scanner mutted completely long before you get any distortion on the Motorola??

In that case you assumption is wrong.

 

[rdale]

"you don't know which is working better."

They are side-by-side, running on identical external whips about 6 feet apart on an otherwise unobstructed roof.

"Maybe the scanner mutted completely long before you get any distortion on the Motorola??"

I really don't think you have ANY idea what you are talking about. I have _never_ heard an occasion where the Mot will do a Max and the Pro96 not have clearer audio. Your assumption is once again proved wrong. There are times when the Pro96 will dropout and the Mot will still get signals, and vice versa, but never the Max Headroom effect.

 

[N_Jay]

"you don't know which is working better."

They are side-by-side, running on identical external whips about 6 feet apart on an otherwise unobstructed roof.

"Maybe the scanner mutted completely long before you get any distortion on the Motorola??"

I really don't think you have ANY idea what you are talking about. I have _never_ heard an occasion where the Mot will do a Max and the Pro96 not have clearer audio. Your assumption is once again proved wrong. There are times when the Pro96 will dropout and the Mot will still get signals, and vice versa, but never the Max Headroom effect.

Well I would be a bit surprised, unless the Moto (or other comercial radio) is not up to snuff.

Two antennas in the same are are usually within 6 dB of each other, but there is no guarentee that they are within 3 dB. That is why you would need to do the test with a calibrated test set up.

Maybe your scanner is particullarly hot, but I would not expect that to be the rule.

If you think I have no ide what I am talking about, then you know less than you THINK YOU KNOW!

as for teh "max Headroom" as you like to call it. I would have to assume you are speaking of vocoder artifacts caused by excessive bit errors. :twisted: But, MAX HEADROOM, might just be the technically correct term. After all who am I to know? :wink:

 

[INDY72]

M M Maybe H H He does, and m m maybe e e he d d don't tt ,..... Technical term bad bits and sound distortion on the voice,... end user slang,... Max Headroom,.. wow someone actually remembers that old show.

 

[rdale]

"Max Headroom,.. wow someone actually remembers that old show."

You betcha... Not that I liked it that much, but it was fun. The entire point of my thread, lost somewhere in the above, is that the Pro96 is more than capable of pulling stuff out that the Motorola isn't and vice-versa.

- Rob

 

[N_Jay]

"Max Headroom,.. wow someone actually remembers that old show."

You betcha... Not that I liked it that much, but it was fun. The entire point of my thread, lost somewhere in the above, is that the Pro96 is more than capable of pulling stuff out that the Motorola isn't and vice-versa.

- Rob

Back to the topic

My point is;
1) that I doubt that in general a scanner is going to decode better than a purpose built radio.
2) the casual (non-calibrated) testing can often give you misleading results

 

[ianw]

One other possibility may be that Motorola's are programmed to ignore
speech when a high proportion of the speech data is corrupt. Officers in the
field aren't going to want to put up with strange fragments of distorted speech
coming from their radio but hobbyists will put up with this if it allows them
to hear more.

I still say though that a Motorola set designed as it is to cover a single
frequency band will be more sensitive and have better strong signal
handling than a scanner.

Ian

 

[N_Jay]

One other possibility may be that Motorola's are programmed to ignore
speech when a high proportion of the speech data is corrupt. Officers in the
field aren't going to want to put up with strange fragments of distorted speech
coming from their radio but hobbyists will put up with this if it allows them
to hear more.

I still say though that a Motorola set designed as it is to cover a single
frequency band will be more sensitive and have better strong signal
handling than a scanner.

Ian

But he is sayiong the opposite (Scanner Clear, Moto broken up)

I think we agree it does not make sense, so there is probably another factor in play.

 

[rdale]

"One other possibility may be that Motorola's are programmed to ignore
speech when a high proportion of the speech data is corrupt."

It does not sound like ignoring... I don't know if you've monitored a digital system with a Motorola rig (it's obvious Jay has not) you will occasionally here the speech "stuck" and it reverberates like the old Max Headroom TV show (hence the name.) When that happens on my Motorola rig the Pro96 simply drops out the audio instead of "skipping" and makes it more usable.

 

[N_Jay]

"One other possibility may be that Motorola's are programmed to ignore
speech when a high proportion of the speech data is corrupt."

It does not sound like ignoring... I don't know if you've monitored a digital system with a Motorola rig (it's obvious Jay has not) you will occasionally here the speech "stuck" and it reverberates like the old Max Headroom TV show (hence the name.) When that happens on my Motorola rig the Pro96 simply drops out the audio instead of "skipping" and makes it more usable.

Nope, I have never head that!!! LOL, yea right.

I was not discussing that you don't get some strange sounding noises out of a vocoder when the BER is high. What I WAS discussing was the assertion that at the SAME signal level the scanner was BETTER at decoding the signal.

You should have heard MIRS (pre-iDEN) in a high signal, low signal quallity area!