DSD+ Comparing Waveforms From 2 Different Scanners

[CanesFan95]

Which one is considered the better wave firm? The first photo on the left is from a Realistic PRO-2006 scanner with a discriminator tap mod. The 2nd photo on the right is a Uniden BCT15X with the same mod. Both mods have the same components: a 10k resistor and a 10 uF tantalum capacitor in series with the tap.

In both photos, the scanner was tuned to the same frequency with the same antenna. The signal strength is very strong on both scanners (the BCT15X shows all the busy bars lit up). I'm trying to figure out why the wave forms are different. The PRO-2006 scanner seems to result in better audio quality coming out of the computer speakers.

 

[br0adband]

The general concept of a waveform is to show a visual representation of a value, whether it's an audio waveform, or an electrical one, etc. In your image above, the right side basically shows nothing of any use at all, barely any peaks which represent the level of the signal being passed across the discriminator tap through the Windows audio mixer - first glance at it shows it's nearly flat which would mean practically nothing at all (flat line = dead, no signal, nothing there to measure basically). When you see peaks and valleys and whatever, that's a very noticeable indication that something is going on, some type of signal is being provided to be measured as opposed to just having a flat line dead input with nothing at all.

The one on the left is significantly stronger and provides more to work with so I have to ask: are you 100% certain that you snapped those screenshots during times when both scanners were actively receiving a signal on the given frequency?

Also, the signal strength received in the scanner's tuner is not necessarily going to translate across to the discriminator tap meaning the baseband audio output is not always (very rarely actually even in spite of the idea of a tap) going to be exactly the same level. The sheer signal itself can account for a lot of variation depending on the way the scanner it handling it internally before being passed to the discriminator (that you're tapping a signal from) and turned into audio output for the scanner's speaker.

In that image, if as much as possible is duplicated - and you said it's the same antenna, same frequency/signal, and I'm going to presume that you're using the same audio input on the computer and not one of several dedicated to each scanner (which is another thing) - and those screenshots are accurate, it would be easy enough to say the BCT-15x just isn't outputting a very "loud" signal on the tap you've created - I'm not saying it's not functional because it might be, but if all the stuff between the scanner (but not including the scanner) and the waveform display - meaning the tap, the cabling, the audio input, the audio levels, the entire pathway - is the same and only the scanner providing the signal is different then obviously the difference lies with the scanner itself.

Now, what I just said about multiple inputs could be relevant as well if that's what you have in action - multiple scanners, with some kind of audio inputs like a physical mixer or multiple line or mic inputs to a computer (like one internal one then another provided by a USB-based sound card/device, etc) could account for discrepancies in signal strength and might be addressed with Windows mixer adjustments. I can't say because I don't know the full actual setup you're using.

But just from a glance, if everything after the scanner is the same and not changed, that means the scanners themselves are just providing a different level of baseband signal to work with from their respective taps.

 

[CanesFan95]

Ah yes, I should have mentioned. Both screenshots were snapped at a moment when someone was keyed and talking. This was on a DMR Tier 3 signal. And, both screenshots were using the same line-in jack on the computer soundcard (this is my only input).

Here's a few shots of the BCT15X mod. I fear I've messed something up and will have to re-do it.

 

[CanesFan95]

I get a similar result with EDACS and Motorola control channels on Unitrunker. Here's a few shots while on a local EDACS control channel with a very strong signal. The first photo is from the PRO-2006 and the BCT15X is the 2nd photo with the smaller waves. They both provide a Health of 100 in Unitrunker, but the performance on PDW and listening to DMR on DSD+ has been worse on the BCT15X.

 

[slicerwizard]

Which one is considered the better wave firm? The first photo on the left is from a Realistic PRO-2006 scanner with a discriminator tap mod. The 2nd photo on the right is a Uniden BCT15X with the same mod. Both mods have the same components: a 10k resistor and a 10 uF tantalum capacitor in series with the tap.

They're both so-so. I assume you're driving a line input, and as you can see, the Uniden doesn't have enough oomph to drive it. The signal from the 2006 is stronger, but the waveform looks distorted to me. Raw audio recordings would be more useful than screen shots though.

In your image above, the right side basically shows nothing of any use at all, barely any peaks which represent the level of the signal being passed across the discriminator tap through the Windows audio mixer - first glance at it shows it's nearly flat which would mean practically nothing at all (flat line = dead, no signal, nothing there to measure basically).

The signal content is quite visible. It's nowhere near flat/dead. Tests show that DSD+ doesn't much care how quiet a signal is, as long as it's there. It's distortions that kill decoding, not low audio.

 

[M105]

Have you got access to an oscilloscope? Comparing the two signals at the tap point will give you some insight into whether you have a problem or if the two scanners simply operate at different levels internally. Scope the BCD with and without your tap removed in case you have created a low resistance "short".

 

[CanesFan95]

Yes, they were both using the line-in jack on the computer soundcard. How do I record a raw audio file? Is there a way to post the recording on here, like an mp3?

I don't have an oscilloscope and prolly know how to use it anyway. I am wondering if maybe I shouldn't have used resisters so that then I'd get a stronger signal. Funny thing, I tried the built-in discriminator on the PSR-800 scanner that I have, and it also has a rather flat wave that looks just like the BCT15X.

 

[M105]

The resistor prevents overloading the scanner circuit. I don't know what the recommended value is for that scanner. I have an old Pro-2006 D-tapped and it puts out more signal than my Pro-2053 but both work fine with dsd+ so I never really worried about it.

 

[CanesFan95]

Just out of curiosity, what does the wave form look like on your PRO-2006 on a DMR frequency while someone is keyed up and talking? Would you mind posting a photo for us to compare?

 

[CanesFan95]

They're both so-so. I assume you're driving a line input, and as you can see, the Uniden doesn't have enough oomph to drive it. The signal from the 2006 is stronger, but the waveform looks distorted to me. Raw audio recordings would be more useful than screen shots though.

Exactly what is it that makes them so-so? How do I tell what is considered a good or bad wave form? What would make them better than so-so? Bigger waves? A more square shaped wave? And what kind of adjustments can I make to improve it?

 

[slicerwizard]

Exactly what is it that makes them so-so? How do I tell what is considered a good or bad wave form? What would make them better than so-so? Bigger waves? A more square shaped wave?

The DMR protocol uses four level signaling; this means that the signal moves between four states (deviations from the signal's center frequency)

There are two smaller deviations, referred to as +1 and -1, and two larger deviations, +3 and -3.

The larger deviations (+/-3) define the envelope of the signal. In the attached image (made from a strong local UHF Cap+ repeater), the red lines show how flat the envelope is.

There are a couple of +1, -1, +1, -1 ... sequences in that signal and one could draw flat lines through all the +1s or -1s as well.

That's a flat, clean signal. That's your (ideal) target. Discriminator taps that truly pass flat audio will give you that.

 

[slicerwizard]

And what kind of adjustments can I make to improve it?

Here's another sample from the same repeater; notice how the signal holds at +1 for a while. The repeater is sending a half dozen or so +1 symbols. During that time, your scanner's discriminator will be generating a fairly steady voltage. That voltage has to get into your PC without being mangled. When you try to feed a line in from a discriminator circuit, you end up overloading the discriminator and that flat line ends up looking more like a ski slope. Now DSD+ has a hard time figuring out that it's seeing a bunch of +1 symbols.

If you can pause DSD+'s waveform display (by right clicking on it) and sometimes see flat sections like that, you may have a fairly good discriminator tap. If you can't find any, something's wrong.

The white lines show all the places where the signal hits +1 and -1; if there are any distortions, DSD+ doesn't see the signal hit those spots and you start losing bits.

In your Pro-2006 image, I can't really draw four flat lines and hit the +/-1 and +/-3 symbols in the signal. The BCT15X signal is too quiet - can't see the symbols, so can't judge it.

A mic input puts far less load on a discriminator tap. But you can't just use a 10k resistor to feed a mic input - the signal will be too hot. For that, you need a voltage divider (two resistors). The ideal values vary depending on the mic circuitry in your PC, which is why some folks experiment with a potentiometer or two and find the values that produce the best looking signal and/or best synthesized audio from DSD+. I've seen mic inputs that like a 47k+10k divider and others that like 5k+5k - you have to experiment.

Sometimes, the problem is a noise filter that's being triggered by the sound hardware's driver software. There may not be much you can do about that.

The cheap $5 USB sound dongles do a good job - their mic inputs and the Windows driver that runs them tend to not molest the signal.

 

[vince48]

excellent explanation slicerwizard

 

[M105]

Just out of curiosity, what does the wave form look like on your PRO-2006 on a DMR frequency while someone is keyed up and talking? Would you mind posting a photo for us to compare?

Everything around me is Nexedge and P25 so I am not aware of a DMR signal I could even screenshot for you. A while back I switched to using two SDR dongles and don't readily have the 2006 in a position to hook back up.

I have several old and new scanners around here, mostly GRE models, and I must say my old Pro-2006 is still the best receiver of the bunch. The display backlight is shot again but that thing has thousands of hours on it with never a problem. I suspect you have no real problem with your BCD and that its signal level is just lower at that stage in the circuit. You might try turning the volume on that computer input up to the max or try using the microphone input instead.

 

[CanesFan95]

Maybe I'll try a USB sound card and see how it works. But most of the ones I'm seeing have a mic jack but no line-in. Will that be a problem, or can I just use the mic plug?

I'm starting to wonder about the idea of doing the tap mods bare, with no resistor or capacitor, just bare wires. Then, you make a little board with a couple pots, a variable capacitor, and 2 3.5mm mono jacks. The board would go between the scanner and the sound card with 2 mono cables. Then you can adjust to your hearts content no matter what scanner you use. That would save you from having to solder extra components inside every scanner and having to guess at the values to use. The board can be easily removed and put on another scanner.

 

[slicerwizard]

A couple of these, say 50k's, on a small board would be a great idea. Adjust them before closing up the case...

Trimpots

 

[Forts]

As mentioned above the mic input is preferred as it puts less strain on the audio source, which makes the little USB audio dongles attractive (plus they are dirt cheap). The only issue I've had with them is the stereo plugs they use are cheap as hell and typically need a lot of wiggling around to make them work. But... you get what you pay for, right?

 

[CanesFan95]

So on the computer soundcard, it's actually better to use the mic input instead of the line-in? Everything I've seen online usually says to use the line-in jack.

 

[M105]

The Mic input is at a much lower level than the typical 1V PP signal at line-in. I prefer it myself but it may require you reduce the volume slider to prevent clipping with too strong a signal.
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On your other idea... I would at least put the capacitor inside the scanner. It passes the audio but blocks DC so it would help prevent damage to the scanner or your external device. No need for a variable capacitor.

 

[slicerwizard]

The Mic input is at a much lower level than the typical 1V PP signal at line-in. I prefer it myself but it may require you reduce the volume slider to prevent clipping with too strong a signal.

Say what? The clipping happens long before the "volume slider".

On your other idea... I would at least put the capacitor inside the scanner. It passes the audio but blocks DC so it would help prevent damage to the scanner or your external device. No need for a variable capacitor.

Did someone suggest a variable capacitor?