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ST Low Pass Filter use on AM
https://forums.stereotool.com/viewtopic.php?t=4458		 Page 1 of 2
Author:  Storm905 [ Sun Jan 06, 2013 3:58 am ]
Post subject:  ST Low Pass Filter use on AM
Orban 9300 manual: "When a station is transmitting with 5 kHz audio bandwidth, the 9300’s 5 kHz lowpass filter can produce audible ringing that some find objectionable. Using the
9300’s bell-shaped HF parametric EQ tuned to 3 kHz can reduce the effects of this
ringing by reducing the boost at 5 kHz by comparison to the 9300’s HF shelving EQ,
which maintains full boost all the way to 5 kHz. Additionally, you can use the LPF
SHAPE control to trade off brightness against ringing."

When setting ST LPF to 4.5 or 5kHz bandwidth for AM, are there any options/approaches to trade off brightness against ringing?
Author:  hvz [ Sun Jan 06, 2013 11:59 am ]
Post subject:  Re: ST Low Pass Filter use on AM
Not yet anyway... Do you hear ringing?
Author:  Storm905 [ Sun Jan 06, 2013 2:07 pm ]
Post subject:  Re: ST Low Pass Filter use on AM
Quote:
Not yet anyway... Do you hear ringing?
I haven't tried ST on an AM service as yet but interested in doing so at some stage, probably once the new multiband compressors are in place. From what I've read, the transmitter and typical AM receivers play a potential role in ringing on narrow bandwidth services that may not necessarily be heard at the output of the transmission audio processor, hence the question.

It was interesting to read in the 9300 manual that they don't seem to recommend running asymmetrical limiting. I know some folks are running ST or BBP followed by an Inovonics 222 (or similar), and if employing Asymmetrical limiting is in fact not desirable, I'm now wondering what they need the 222 in the chain for.
Author:  hvz [ Sun Jan 06, 2013 2:35 pm ]
Post subject:  Re: ST Low Pass Filter use on AM
O, I was actually planning to include an asymmetric clipper later. Why do they advise NOT to use one?
Author:  Storm905 [ Sun Jan 06, 2013 10:35 pm ]
Post subject:  Re: ST Low Pass Filter use on AM
Quote:
O, I was actually planning to include an asymmetric clipper later. Why do they advise NOT to use one?
Orban:
'While the physics of carrier pinch-off limit any AM modulation system to an absolute
negative modulation limit of 100%, it is possible to modulate positive peaks as high
as desired. In the United States, the FCC permits positive peaks of up to 125% modulation. Many countries have similar restrictions.
However, many transmitters cannot achieve such modulation without substantial
distortion, if they can achieve it at all. The transmitter's power supply can sometimes
be strengthened to correct this. Sometimes, RF drive capability to the final power
amplifier must be increased.
Voice, by its nature, is substantially asymmetrical. Therefore, asymmetrical modulation was popular at one time in an attempt to increase the loudness of voice. Traditionally, this was achieved by preserving the natural asymmetry of the voice signal.
An asymmetry detector reversed the polarity of the signal to maintain greater positive modulation. The peaks were then clipped to a level of -100%, +125%.
OPTIMOD-AM takes a different approach: OPTIMOD-AM's input conditioning filter
contains a time dispersion circuit (phase scrambler) that makes asymmetrical input
material, like voice, substantially symmetrical.
OPTIMOD-AM permits symmetrical or asymmetrical operation of both the safety
clipper and multiband distortion-canceling clipper. Asymmetrical clipping slightly increases loudness and brightness, and can produce dense positive peaks up to 125%.
However, such asymmetrical processing by its very nature produces both odd and
even-order harmonic and IM distortion. While even-order harmonic distortion may
sound pleasingly bright, IM distortion of any order sounds nasty.
There is really nothing lost by not modulating asymmetrically: Listening tests easily
demonstrate that modulating symmetrically, if time dispersion has been applied to
the audio, produces a considerably louder and cleaner sound than does asymmetrical modulation that retains the natural asymmetry of its program material.
Some of the newer transmitters of the pulse-width modulation type have circuitry
for holding the carrier shift constant with modulation. Since artificial asymmetry can
introduce short-term DC components (corresponding to dynamic upward carrier
shift), such carrier shift cancellation circuitry can become confused, resulting in further distortion."
Author:  W2xj [ Sun Apr 27, 2014 1:37 am ]
Post subject:  Re: ST Low Pass Filter use on AM
Bob Orban wrote that about 35 years ago. His first AM Optimod had a phase flipper to maintain positive peaks. The problem with phase flippers is that they are annoying to on air talent if they are monitoring themselves off air in real time. The stuff about transmitter capability went away decades ago. Most modern transmitters can easily reach 150 to 160% positive modulation.

To my knowledge, nobody has ever manufactured a processor for assymmetrical modulation. To properly accomplish asymmetry one must start at the individual microphone inputs and assure the peaks are going in the correct direction. Then every compression and limiting section in the processing chain that employs peak detection must either have positive peak detection disabled or at least be adjusted to a higher threshold. This can be done if attention is paid to every detail. I have done it at numerous stations and it does in fact sound better and more open than phase scrambled audio.

All that said, the usual method is to just clip the negative peaks until the positives reach the desired level.
Author:  hvz [ Sun Apr 27, 2014 10:25 am ]
Post subject:  Re: ST Low Pass Filter use on AM
This is a reply to a much older post - Stereo Tool supports asymmetric limiting and clipping now.

But I'm curious about the ringing. At least theoretically, it should be present with certain steep lowpass filters, including those in Stereo Tool. I have never noticed it myself - although I just did some tests and now I think I do notice it. At lower latencies there's less riniging - and at least with low settings for the lowpass frequency it seems to sound better with lower latencies. I do know how to get rid of it. So, if anyone else hears it, let me know.......

Edit: Lol. Ok. Never mind. I had already implemented the filter to remove any ringing. :)
Author:  hvz [ Tue May 20, 2014 4:22 pm ]
Post subject:  Re: ST Low Pass Filter use on AM
I wonder if anyone is planning to try to use Stereo Tool for C-QUAM stereo? Based on what I read, what I would need to do for that is to clip/limit the L+R and L-R signals separately, but also clip/limit one side of the L and R signals. Which should be easy enough...

I just found the US AM specs:
http://www.nrscstandards.org/SG/NRSC-1-B.pdf
Looks like the current 75 us pre-emphasis curve is not precisely correct, the highest part must be attenuated a bit.
Quote:
The analog AM audio bandwidth transmission standard is specified in Figure 4. The audio envelope input
spectrum to the AM transmitter shall be -15 dB at 10 kHz, smoothly decreasing to -30 dB at 10.5 kHz,
then remaining at -30 dB from 10.5 kHz until 11.0 kHz. At 11.0 kHz, the relative amplitude shall
be -40 dB, smoothly decreasing to -50 dB at 15 kHz. Above 15 kHz, the relative amplitude shall remain
below -50 dB. The reference level is 1 dB above a 200 Hz sine wave at 90% negative modulation.
Meaning that a lowpass filter at 10 kHz if sufficient, IF I guarrantee that filtering starts below 10 kHz (there's always a rounding issue, and it needs to be rounded down). The filtering in Stereo Tool is less steep at lower latency settings; rounding down will always keep the maximum frequency below 10 kHz.
Author:  hvz [ Tue May 20, 2014 5:31 pm ]
Post subject:  Re: ST Low Pass Filter use on AM
Ah good. The lowpass filter was already protected against rounding errors. That leaves a few things to be done:
1. Change the pre-emphasis shape - NOT YET TESTED (check that FM is still ok and that AM is also ok, slightly less highs)
2. Put the pre-emphasis setting in the AM panel
3. Put a headroom slider in the AM panel (instead of abusing the Post Amp slider)
4. Turn AM stuff off if FM is enabled (?) but what about asymmetrical clipping/limiting for FM? -> Extra switch in Expert mode
5. Add pre-emphasize output button for AM pre-emphasis
5. Maybe - if people ask for it - add a C-QUAM mode.
Author:  bob53bob [ Thu May 22, 2014 3:15 pm ]
Post subject:  Re: ST Low Pass Filter use on AM
Quote:
but what about asymmetrical clipping/limiting for FM? -> Extra switch in Expert mode
No asymmetrical modulation for FM. That would result in over modulation or the need to reduce the modulation to the highest peak resulting in loudness loss.
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