Any dc blocking caps in the audio path between the audio processor's output to the tx modulators input cause it to have a high pass filter reponse.
This low frequency h/p filter response causes lf tilt because the series dc blocking caps start to charge/discharge on every low frequency cycle of deep bass.
This causes the flat topped clipped and filtered bass waveform of the processor to get tilted.
This gets difficult to explain, but if you were to inject a squarewave of say 50hz into a h/p filter, the series cap would get partially charged on one half of the cycle, and then when the input waveform changes polarity, the cap discharges this charge which gets added to the following half of the waveform, so increasing the peak initially before decaying, and the cycle continues.
If the series dc blocking caps were large, this would minimise this effect.
When looking on a scope, the the tilted top waveform produced has overshoot on the leading waveform, and equal but oposite undershoot on the same top lagging waveform.
Regarding the AMT5000 bypass mod, only the balanced audio inputs will work, the unbalanced audio inputs won't work, as these are routed direct to the processor chip, which as been bypassed !
If you are using the AMT5000 sited at the base of an external antenna, balanced audio by cat 5 cable is the way to go anyway.
Paul.
I was sincerely skeptical that audio improvement of quality part 15 AM transmitters (from SSTran) was even possible, but the information presented has definitely shown that there's more going on than most would think or know about.
The development work shared by BOARDMAKER is a substantial contribution to the hoard of technical knowledge documented on this website.
Print it and tuck it into the Part 15 File Folder!
behind [[Log-in to view link]] until the "initiates" have been hazed sufficently.
I loved the sound my SSTran AMT3000 produced when I had it, admittedly the Rangemaster does blow it out of the water when you have pro processing gear.
The Rangemaster will typically always have the upper hand due to its adjustable power output which I don't believe the Procaster has. Both transmitters operate beautifully however and you can't go wrong with either. Another one to look at is the Grain industries transmitter which costs about the same and also has built in processing.
From what I've read about the Grain Industries transmitter -- it has processing in the form of modulation control but that's it, making sure to keep modulation at 95%. And there seems to be no adjustment for this modulation control in the transmitter.
The Procaster allows adjustable compression, modulation, has built in limiting, and the whole system can be easily bypassed if you choose to use outboard processing.
There is a clear difference in useable range between 95% and 125% positive modulation. Some would argue there is increased distortion, but I've been listening to my station at 125% for three years and have yet been able to detect it compared to local commercial stations at 100%.
That said, the Grain industries unit looks to be a well built, easy to operate and install transmitter.
TIB
I still need to do tests for symmetrical and asymmetrical modulation.
Orban claims that symmetrical mod with phase rotation has no disadvantage over assymetrical mod without phase rotation.
I have done speech tests with phase rotation in and out before a clipper, and i have been able to do 3db more clipping with phase rotation, compared to no phase rotation, for the same perceived distortion.
Tim, i wonder how dynamic carrier control would fly in part 15 legality ?
Paul.
I've wondered which would be better, loudness versus quality, between symmetrical and asymmetrical to +125 percent. My personal feeling is that phase scrambled audio at +/- 100% is the safest thing to use.
There are different schools of thought, some are concerned about radios distorting at high levels of negative modulation because of cheap detectors in them. Some of the more recent vintage transmitters don't even like modulation past 90-95% negative. I think the problem is pretty much solved now, but we have a few decades worth of AM broadcast transmitters at stations that should watch negative peaks.
That could have been one of the things that led to using more asymmetrical mod, to keep audio up by going more positive.
Then too, if you'll even hear the added volume would depend on how much you compress, limit and clip already. I'd think if you were broadcasting natural dynamics, you probably won't notice the difference between peaks going to 100 or +125, but extra db on brick wall audio should be heard.
The only problem then is that so many peaks being treated to the asymmetrical processing, the distortion would probably be Super Bad!
There must have been some studies on this, at around 1970 when the FCC was considering the 'limiting' of positive modulation. Apparently before then, there was no 'limit' on the positive peak modulation, so stations mightt have been doing something to trigger the FCC on it?
Modern CBers are really into asymmetrical mod these days, the guys who talk on the nationwide channels. 300% mod is not untalked about, and they're building 12-pill wide dynamic range superamps for CB. You should see the sine waves on their scopes, with big troughs on the negative peaks, "Look at that signal, you'll see the carrier is never cut off!"
I've never seen data on using dynamic carrier control in Part 15 AM before, and if it would be okay. There was a transmitter from Panaxis called, I think, AM 100? Whatever it was called it was crystal controlled and used carrier shift to increase power when the modulation would peak up. I'm guessing that the no modulation carrier power was 100 mW..
I was having a discussion with a radio friend about that transmitter after we both saw it written up in Panaxis' newsletter. I thought, "Ah, clever way to get more milliwatts!" and my friend thought it was because that transmitter was meant to feed a 4 watt linear amplifier for power line transmission, so that the transistors wouldn't overheat and save power.
It might be something to look into again, since commercial broadcasters looking into and applying several different carrier control strategies now.
For experimenters who do not want to use transmitter on-board audio processing, and do not have a hardware processor available, there is an amazing product by Hans Van Zutphen called StereoTool which comes either as a DSP Plugin for Winamp or as a stand-alone software processor.
StereoTool has an AM section that allows selecting and adjusting asymetrical modulation and NRSC pre-emphasis and it does the job very effectively.
I used it until I realized that sending asymetrical modulation simultaneously over audio streams and on FM transmitters sounds pretty bad, and I did not have enough means to manage output distribution to serve all needs, but for exclusive use on AM StereoTool is a way to go.
IIRC there are no actual restrictions on mode of broadcast for part 15 devices aside from the universally forbidden spark gap transmitters.
So theoretically you can run whatever mode you want, be it AM, SSB, FM, DAB, ect.
Interesting info about the Grain transmitter Tim.
I also broadcast at +125%, I can confirm a difference even between 100% and 125%. I haven't noticed any distortion or degraded audio at all, if anything running at 125% actually sounds BETTER.
IIRC back in the 60s stations were running 130% and higher causing the FCC to limit positive peaks to 125%.
Does anyone here have any experiance with the Grain transmitter or know someone who owns one? I would be interested in his or her's first hand experiance.
There is no limit on positive modulation for Part 15 AM as long as your transmitter can handle it and you don't cause interference to other stations. I remember Panaxis advertising one of their AM microbroadcast transmitters as being capable of +300% modulation!
But of course, you still need to keep your negative modulation below -100% or else you'll splatter all over the band and produce distorted audio. Many licensed AM stations limit their negative modulation to -95% due to the poor design of most AM receivers causing a large increase in distortion if you exceed that level.