Audio observations

[ki4nr]

I've been listen on the bands and my observations on majority of AM signals is the audio modulation levels are to low & not much in the way of fidelity. Most of the stations sound like they are averaging about 40 to 60 percent audio with maybe occasional peaks on the scope of 100 percent. This is fine for big strapping signals but when your in lower signal areas you need to more modulation.

With all the inexpensive easily available audio processing equipment, why is this not being used ?? Run a compressor and limiter to bump up the audio and use an EQ to get the fidelity that AM is capable of. Back when I was on the bands I was using a FT-102 & Heathkit amp with a decent mic, a good EQ, budget Alesis nano compressor directly to the modulator and the audio on that setup had super fidelity, clean and powerful sounding. Sure it was inefficient being low level and linear amped, but it was Hi Fi audio.

I was listen to Don K4KYV a few nights ago on my Kenwood TS-870 which is known for Hi Fi DSP audio & polk audio 5 monitors, his audio was superb, Just enough compression and EQ ing, also Steve, WA1QIX has great audio and plenty loud too. There are a few others that have things setup right.

I think a tutorial is needed on " how to have great audio on AM with low dollar processing equipment "  from what I been hearing on the bands it's needed.

John KI4NR

[N1BCG]

This might inspire discussion...

Pro Sound On A Budget

Phone operators of all modes using everything from classic tube rigs to the latest solid state marvels with DSP technology strive to get the most from their transmitters to combat the challenges presented by propagation and interference. “Loud and Clear” is more than an expression, it’s a crucial goal for effective communication and the two terms are neither exclusive nor just for those with large budgets.

It’s also important to keep in mind that human ears, not meters, software, or oscilloscopes, are what judge the intelligibility of our audio, so whatever can be done to contour our sound toward that goal will result in effective communication.

Accomplishing the task

Fortunately, it doesn’t take much to make a big difference. Consumer grade stereo audio processors and equalizers are affordable (compared to the thousands of dollars that broadcast processors cost) and can be fully utilized with both left and right channels of each unit set up to handle four key audio processing functions: 1) maintaining high average modulation, 2) emphasizing the audio frequencies that are heard most easily, 3) keeping transients and peaks in check, and 4) packing that all into an efficient transmitted signal.

I tested this using older equipment that I had on hand but the concepts described here would apply to most off the shelf stereo compressors and equalizers you can find in music stores or online. For compression and limiting, I use a DBX 166XL compressor/limiter and a Yamaha GQ2015 equalizer. I particularly like them for their flexibility and balanced XLR connections although the latter isn’t critical.

The audio path is simple and routes through the left channel of the compressor/limiter, then the left channel of the equalizer, the right channel of the compressor/limiter, and finally the right channel of the equalizer which feeds the transmitter input.

(Figure 1.jpg)(Figure 2.jpg)

Here are the four stages with an explanation of how and why each is set to accomplish the desired function:


1) Compressor/Limiter (Left Channel)   (Figure 4.jpg)

Role:      Audio Gain Control
Benefit:    Maintains consistent modulation for varying input levels due to normal
                fluctuations in voice level or moving closer or further from the microphone
Settings:    8dB Gain Reduction on voice peaks, 2:1 Compression Ratio,    Medium
                Speed Attack, Med Speed Release, No Clipping or Limiting (if an option).

Discussion: These settings optimize perceived volume by keeping average modulation levels high, but not fatiguing. Since our ears rely on changes in volume as cues for intelligibility, the audio should be loud but not crushed. The settings listed will reduce dynamic range in half (effectively doubling your perceived volume) while maintaining important contrasts in level. A more open or dense sound can be accomplished simply by adjusting the release time control. The midway adjustment of the attack time prevents pops and other audio peaks from reducing gain.


2) Equalizer (Left Channel)

Role:      Pre-emphasis (high frequency boost)
Benefit:    Increases intelligibility by boosting audio frequencies that the ear is
                most sensitive to
Settings:   See discussion

Discussion: Many receivers begin to roll off frequency response above 2 kHz, so to compensate and to sound less "muddy", the "presence frequencies" are emphasized. A lot of research went into developing the NRSC equalization curve for broadcasters and I found that it works very well to emphasize frequencies the ear is most sensitive to and improve the signal to noise ratio. Why reinvent the wheel? The following are approximate settings since equalizers vary in design and band assignments, but the sliders beginning around 1kHz should be set for a smooth increase to follow the NRSC Pre-Emphasis curve, or close to it.


3) Compressor/Limiter (Right Channel)   (Figure 4.jpg)

Role:      Peak Limiter
Benefit:   Allows for maximum modulation and prevents peaks from causing splatter
                or even damaging transmitter or amplifier components
Settings:   -3 to -5dB Gain Reduction, inf:1 Compression Ratio, Fast Attack, Fast
                Release, Peak Clipping (see discussion)

Discussion: The sole purpose of this stage is to eliminate peaks that force modulation to be set low or cause splatter from overmodulation. Excessive peaks can permanently damage modulation transformers or RF components so setting this stage correctly is very important.

I’ve found that manufacturers approach peak control in a variety of ways ranging from quickly reducing gain (limiting) or cutting them off (clipping). Since each channel usually has a separate compressor and limiter, it’s the latter that will be doing most of the work for this stage.

The limiter in the DBX 166 XL is labeled “Peakstop Limiter” and aggressively affects the waveform much like a clipper. An LED indicates that this circuit is active and the control should be set so that this only flickers with audio.

By contrast, the limiter in the Symetrix 565E is more like a true limiter or soft clipper. This unit features four LEDs showing limiter gain reduction, which should be set for 3 to 6dB with audio.


4) Equalizer (Right Channel)

Role:      Low Pass Filter (LPF) for bandwidth limiting
Benefit:   Limits excessive bandwidth generated from limiting/clipping while passing
                useful audio frequencies
Settings:   Flat to 4k, -40dB above

Discussion: Limiting bandwidth is necessary for many reasons, but for the purpose of this article, it’s primarily to filter out the harmonic energy generated by the (final) limiter/clipper stage. Harmonics are generated any time the peaks of a sinewave are sharply altered, and while limiting and clipping in particular do just that, it’s important to reduce the energy beyond what is needed or heard.  

Some things to check once everything is wired and ready to try…

* Proper setting of the output levels for each stage is important. Too low a level introduces noise while too high a level will overdrive the next stage and cause distortion. It would be expected to add a bit of output gain to the first stage to make up for the gain reduction of the compressor.

* The switch that couples the gain reduction bus of each channel of the compressor must be set to disable the coupling and allow the left and right compressor/limiter to function independently. This switch can be labeled Stereo Strap, Dual Mono, or Stereo Couple depending on the manufacturer.

* Since the right channel of the equalizer is the last stage before your transmitter, the level control should be set to suit the input of your transmitter. If you have a microphone level input, you will need to use an attenuator to reduce the line level audio output of the equalizer. Simply turning down the equalizer level may work, but you will introduce noise into your transmitted signal.

* This arrangement provides complete audio processing for transmission. Any equalization or compression within the transmitter (DSP, analog, etc) should be set to OFF.

You can try this with other models of compressors and equalizers as long as the basic goals are met. Some processors combine a compressor and equalizer such as the popular Symetrix 528 series. While this unit and similar were designed for recording studio use, they can be put to work in your audio chain as long as a limiter is placed between them and your transmitter. The reason is that the equalizer follows the compressor in the audio path. Whatever leveling the compressor section does will be defeated by boosting any of the equalizer bands. A final limiter of some kind is critically important if using this type of processing.

This article is in no way a complete discussion of audio processing. Instead, the intent is to assist operators with a starting point for experimentation using commonly available equipment.

[WBear2GCR]

Most of the "regular" AM ops up here in the NE have very good audio.

Perhaps the audio down in "4 land" is lacking??

Nobody is going to get "good audio" out of a stock transceiver, no matter what outboard boxes
someone uses...

Behringer makes usable and very cheap when new, even lower when used compressor limiter
and EQ boxes, fwiw...


                          _-_-bear

[DMOD]

Most of the "regular" AM ops up here in the NE have very good audio.

...Nobody is going to get "good audio" out of a stock transceiver, no matter what outboard boxes someone uses...
                          _-_-bear

Exactly. Many times I hear someone say, "I want better audio but I want to keep it stock."

Well, those are two conflicting criteria.

Keep it stock and sound crummy, with bassy 65% modulation, or go in and modify the speech amplifier and modulator.

Trying to use outboard equalization and compression to force audio in a less than adequate modulation system is like trying to force glue through a No. 34 medical needle.

Most of the AM ops I QSO with have good clear audio and high modulation percentages.

I would interested in knowing what is the OP's receiver bandpass.

Phil - AC0OB  

[KB2WIG]

J

You may wish to click on   " East Coast Sound ",  on the left part of this page.

Its the right thing to do.

KLC



 ps..  welcome t o the board........

[KL7OF]

I'm using this setup as per the instructions from N1 BCG..... that's a Behringer MDX 2000  2 Channel compressor limiter and a biamp 2-channel equalizer.... I'm using the little tube pre on top for the Phantom voltage for the  MXL 770 microphone
The initial settings seem to be pretty good ....I ran it into a stereo to listen to it and tweaked it.... I'm going to try it on the transmitters later.
These boxes are garage sale and high school band auction finds...cheep........

[WD8KDG]

My two cents:

It has been said before by others, "Running AM without an oscilloscope is like driving a car at night without headlights." All of my receiving is now by R390/A's and the IF output on the back of these receivers goes to an o'scope. A high percentage of signals heard are not well modulated. Scopes are cheap.

Issue #2 is all low frequency bass. Might sound gud in the same town, but add a couple hundred miles, propagation, RFI, and what is heard on the other end is mud...……..all mud.

Two of the stations heard well and understood just about every night during winter conditions here in the PNW are WA1HLR and K4KYV. Well balanced audio and no mud. Want to be heard/understood/have a nice QSO, leave out the mud.

Craig,

[Opcom]

I get consistently good reports of full and clean/clear audio with this. Includes one of those cheap Behringers. The Altec is just used as a preamp without the plug-in mike preamps some use instread of input transformers. The rest of the gain is made up in the Behringer. It's very simple and There's no need here for EQ, but for others it could depend on the voice and microphone. There is a polarity switch. I never use more than 120% positive mod.

Those who not use vacuum tube speech amplifiers to drive high level class B modulators can disregard the 150W speech amp and the rest below. I like old fashioned ways and tubes, so..

Those who do use vacuum tube speech amplifiers can take note that the amp is 5x the power rating required by the class B grids. 40% of its rated output is swamped by a resistive load and this protects the speech amp as well. 30% goes to the transmitter, and 30% is reserve so the load is light. The drive regulation is therefore excellent and it never sounds distorted, muffled, or unclear according to reports.

The voltage switch on the speech amp output is set at the lowest voltage at which there is enough peak grid to grid drive to the modulator tubes to make 100% mod. That varies with modulator and RF stage plate voltages and power levels, so it's nice to have. The '500 Ohm line' to this transmiter is just a namesake and is more like 60-70 Ohms due the grid transformer ratio.

Later I want to dump the Behringer and Altec and go all tube. For now they are a connivinance.

[KL7OF]

https://www.nch.com.au/software/radio.html
Has anyone tried this?