AM Hi-Fi Audio Processing

AM Hi-Fi AUDIO PROCESSING

Amplitude Modulation... That old but still widely used mode for broadcasters and Amateur Radio operators alike! Until recently, AM had always been a mode that supported fidelity better than the typical 2.4k SSB rigs... With proper processing, AM can be a full, rich mode of operation!

---

Introduction

Amplitude Modulation
AM may not be the most efficient mode of communication or bandwidth usage but it is still used on the HF bands and can have full and natural sounding audio if set up accordingly. A well controlled, fully modulated carrier with sufficient transmitter bandwidth and audio processing can be a beautiful thing... Especially if you have been used to listening to narrow SSB signals.

Regardless of how you are achieving your AM (High-Level or Low-level), this page is primarily targeted to those who are interested in AM Hi-fi audio processing and the reasons for its employment. I choose to just cover some fundamental reasons for external processing and a few of my favorite processors that in my opinion do a wonderful job at taking command of the pre-transmitter audio. This page is certainly not a highly technical "white paper" of any kind, so please forgive my simplistic review of the subject matter.

AM Hi-fi Audio
Hi-fi Audio processing and AM were made for each other! In fact, there are not any serious commercial AM broadcast stations that do not employ some form of audio processing. Most commercial broadcast stations use Orban, Omnia or Aphex AM audio processing of some kind, and a few on a lower budget using Inovonics, some old used CBS AM processing or even home-brew processing.

In any case, some form of processing is used and there are good reasons for it... I will try to address generically the processing equipment and reasons for its use on this page.

---

AM Transmitter Hi-Fi Audio Processing

Why Do We Need Transmitter Audio Processing for Amateur Radio AM?
I suppose if we were to get philosophical about what we really need for AM, the answer would be "Nothing". But then by the same philosophical standards of inquiry, do we really need AM at all? Aside from emergency communications, do we really need amateur radio? Again the answer would be "no"! Okay, philosophy aside, let me rephrase the question lest I be misunderstood...

"Why Do We Need Audio Processing for "Great Sounding" Amateur Radio AM?"... This is an entirely different question and one that I will attempt to address as we move along. But the simple answer is this: Bare AM electronics does not support the natural dynamics (audio amplitude) of human speech well enough to contain these dynamics which cause all forms of distortion. Nor does it support a good flat frequency response necessary for accurate reproduction of human speech. For the longer, more elaborate answer and one that is in context with this page, please keep reading.

Audio Quality Checklist
There are several factors that will determine the overall quality of our transmitted AM signal. They are:

This is of course just an arbitrary example and these bandwidths could be anything. But the point is that the occupied RF bandwidth of standard AM (A3E) occupies twice the bandwidth of the actual audio. So, if a 6kHz I.F. filter is used, the resulting audio bandwidth would only be about 3kHz. Hardly worth the effort as far as I'm concerned since any modern SSB rig can easily accomplish this using 1/2 of the occupied RF bandwidth for the same audio response.

In the example above, a 10kHz I.F. Filter was used resulting in 5kHz of audio response... This is a huge difference in terms of reproducing more natural high frequencies found in human speech. Just remember, that when selecting I.F. filters, remember the filer bandwidth / 2 rule of audio response.

"Flatness" is a term that relates to even or "flat" amplitude across an audio spectrum within 3dB. In our context here, a "flat" device is one that can reproduce the original signal without altering the amplitude of that signal at any given frequency within the desired passband. For example, if a pure "white noise" source, where all frequencies are even in amplitude, is fed into a device like a stereo amplifier, then the stereo amplifier is said to be "flat" if its output faithfully reproduces the white noise source with the same amplitude at any given frequency. See the graph below (Figure 1) for some different flat and non-flat measurements.

The spectral graphs above are:
White Noise where audio amplitude it equal across the spectrum
Pink Noise where audio amplitude is descending by -3dB/Oct across the audio spectrum
Brown Noise where audio amplitude is descending by -6dB/Oct across the audio spectrum.
Non Flat Radon Receiver Noise where there is "roll-off" on both sides of the audio spectrum.

Note that White, Pink and Brown noise are all versions of flatness since the amplitude decreases in a constant linear fashion unlike the random receiver noise from a non-flat receiver. Notice that except for the random non-flat receiver noise, all of the other plots are a relatively straight or a "flat" line. In the context of our discussions here, we will reference flatness to white noise where all frequencies of concern are represented equally in amplitude within +/-3dB.

With this in mind, very few (if any) AM transmitter would be considered a flat device. There are several reasons for this but without going into that, rest assured that virtually EVERY AM transmitter needs an equalizer to restore flatness or at least something that is close. This is where "Equalization" gets it name. To "Equalize" is to restore the output signal to something that looks like the signal that was fed to the input of a device such as our voice. To put it another way, we want to make the output audio frequency amplitude of the passband equal to what the source audio frequency amplitude was before it was distorted. I will cover more on equalization later in the "The Proper Balancing of Audio Amplitude Equalization Throughout the Voice Spectrum" section.



(2) The Audio Distortion Properties of the Transmitter
The transmitter's ability to reproduce audio with less than 1% THD (Total Harmonic Distortion) with low IMD (Intermodulation Distortion) figures will be very difficult to achieve. This is beyond the scope of what I am writing about here, but every effort should made to keep your transmitter as clean as possible! Also keep in mind that a transmitter that cannot reproduce audio flatness would also be considered a form of distortion, since the resulting output audio frequency amplitude is a distortion of the original audio source.

The audio processing that will precede the transmitter may even aggravate the transmitters distortion specs, so again, great care should be taken when adjusting and aligning your transmitter and pre- transmitter processing.

Know your transmitter's final PEP rating and keep your carrier power at no more than 1/4 of this or less! I will explain more about this later when dealing with "The Percentage of Modulation Used".



(3) The Audio Characteristics of the Microphone
Your microphone selection is not as critical as the transmitter's bandwidth capability, but still worth some consideration. After all, if a microphone cannot reproduce frequencies below 200 Hz or above 3kHz accurately, all of the EQing in the world cannot make up for its deficiencies and the AM audio that you transmit will be limited before it ever has a chance at the transmitter!



(4) The Percentage of Modulation Used
This is a major issue with the AM mode. Since AM is inherently an Amplitude Modulated mode by design, the carrier modulation will be directly proportional to the "Loudness" of the perceived audio relative to the carrier strength.

A nice way to monitor modulation percentage is the use of a "Mod-Monitor" (Modulation Monitor) that reports both positive and negative peak values. An oscilloscope can also be used for this purpose although it is a little harder to interpret positive vs. negative envelope peaks.

If you are looking for a formula designed to calculate positive modulation percentage, and you know the "PEV" (Peak Envelope Voltage) vs. the unmodulated carrier voltage via an oscilloscope, the following formula (Figure 3) was provided by K2WS:



Where:
PEV = Peak Envelope Voltage
Carrier Voltage = Unmodulated Voltage of the Carrier

When an carrier is modulated by 100%, the voltage will double and the power output will quadruple relative to the carrier output.

If we produce a 100w carrier, and the modulation level is at 100%, the peak envelope power is 400 watts. But, the peak envelope voltage max is twice the peak envelope voltage of the carrier alone. So, for example; if the carrier alone produced 20v RF peak to peak, then 100% positive modulation will produce 2 x 20v which equals 40v peak to peak voltage.

Since power changes with the square of the voltage, then the power during 100% positive modulation will increase by the square, so the power increases 2 squared or 4 times! So, 100% positive modulation will produce 2 times the RF voltage and 4 times the RF power.

Using an oscilloscope to measure modulation percentage is achieved as follows:

Set the AM carrier power and adjust the scope to show 1division above and below the center line. (Figure 3) Modulate the carrier and note the increase in the scope's amplitude... If the scope shows 2 divisions above and below the center line on modulation peaks, (a doubling of the voltage amplitude) you have achieve 100% positive symmetrical modulation on the peaks and the power output will be 4x of the carrier. Notice also that at 100% modulation, the valleys between the peaks will close in to the center line to the point where the envelopes are just "kissing" each other... (Figure 4) When undermodulated, the peaks will not reach 2 divisions above and below the center line and the valleys in-between will not close all the way in to the center line. The resulting audio will be light and weak relative to the carrier. (Figure 5) When overmodulated, the scope's amplitude peaks will exceed 2 divisions above and below the center line and the valleys to the center-line will be overlapped looking like figure 6a with little beads in-between for balanced modulators (low-level modulation) and will look like figure 6b when using plate modulation (high-level modulation). This indicates a distorted envelope. (See figures 6a and 6b) See Illustrations below provided courtesy of CleanRF Technologies:

Figure 3

Scope Calibrated for +1 and -1 Divisions
With Full Unmodulated Carrier Applied

Figure 4

AM at 100% Symmetrical Modulation


Figure 5

AM Undermodulated


Figure 6a

AM Overmodulated (Low-Level)


Figure 6b

AM Overmodulated (High-Level)


Figure 6c

AM Asymmetry - Correct Phase (Pos 110%, Neg 95%)


Figure 6d

AM Asymmetry - Inncorect Phase (Pos 95%, Neg 110%)

Also see the "Scope Your Audio Modulation" page for details on an RF Sampler and RF Demodulator and their uses with an oscilloscope, or refer to the information later on this page in the "AM Transmitter AF / RF Monitoring".

An oscilloscope is the best way to measure voltage amplitude. However, if you happen to have a good quality Peak-Reading wattmeter, the formula can be expressed to accommodate PEP instead of PEV as follows:



Using the formula above in an example, if we start with a 100w unmodulated carrier, and with modulation we measure 400 watts PEP with our peak reading watt meter, the math would solve as follows:

Pos Mod % = [(square root of 400 - square root of 100) / (square root of 100)] x 100
Pos Mod % =[(20 - 10) / (10)] x 100
Pos Mod % = [(10) / (10)] x 100
Pos Mod % = [1] x 100
Pos Mod % = 100%

If the carrier is under-modulated, the resulting audio will be light and subdued. On the other hand, if the carrier is over-modulated (above 100% negative peaks), distortion will quickly set in. If the negative modulation peaks can be limited to under 100%, then the positive peaks could be above 100% without distortion! This can ONLY be achieved with asymmetrical modulation. Symmetrical modulation, like a pure sine wave, can NEVER exceed 100% modulation without distortion. Because the human male voice tends to be asymmetrical, we can take advantage of the audio phase orientation where the positive peaks are greater than the negative peaks.

This is where "Asymmetrical Peak Limiting" built into modern AM processing comes in handy! Again, the average male voice is asymmetric in nature. This means that when examined on an audio spectral analyzer, the amplitude peaks will be slightly higher on one side of the plot as opposed to the other. This is why proper "Phasing" of the audio is critical! We always want the positive peaks to be higher than the negative peaks in amplitude since the negative peaks cause distortion when modulated above 100% at the transmitter. There are devices out there called "Phase Rotators" and "Phase Scramblers" that keep the the voice phase positive at all times. However, these devices are not really needed if just one mic and one voice is used. Once the correct phase is established, it will rarely ever change with just one voice and microphone.

With modern AM processing where built in asymmetrical peak limiting is employed, we can manage the negative peaks at about 95% while letting the positive peaks travel well above 100% while still satisfying the transmitter with clean output. It's like having your cake and eating it too! Of course, like anything else, there are some purity issues and tradeoff when using real aggressive peak-limiting. There will always be a fine balance between low distortion and aggressive peak limiting. The trick is finding the balance that will satisfy both sides of the equation - Loudness vs. Cleanness. A little bit of peak limiting will go a long way, so don't get too carried away!

For those on a tight budget like me, I personally like the Inovonics Model 222 AM processor. This AM processor has a pre-emphasis network, a low-pass filter (either at 9.7kHz or 5kHz), a tight asymmetrical peak limiter, and filter overshoot compensation. Also, the "Positive Peaks" control can be adjusted for a maximum of 130% positive peak modulation relative to the negative peaks! All features are bypass switchable You can find the 222 at Broadcast Supply Worldwide (BSW) for around $550.00.



(5) The Proper Balancing of Audio Amplitude Equalization
As mentioned earlier, equalization helps restore some flatness to a not so flat signal. The flatter the transmitter can be made to operate naturally, the less need for external equalization processing. Ideally, if a transmitter were perfectly flat within the desired audio passband, say from 100Hz up to around 5~7 kHz, and the microphone being used is flat in the same passband, then theoretically no equalization would be required other than for personal persona and effect. But modifying or building a transmitter with flat audio characteristics can be quite a challenge in some cases. So audio equalization can be a very useful tool in restoring some degree of flatness to a less than flat transmitter.

The RF Demodulator and Modulation Monitor makes it possible to monitor the transmitter's demodulated AM audio via a 1/4" TRS jack. This audio output is suitable for either headphones or the line-level input of your stereo amplifier or mixer.

These boxes are a much better way to monitor your AM demodulated audio signal than any external antenna method since the monitoring occurs in-line instead of from a constantly varying external RF signal. I also find that there are no proximity 60Hz hum influence problems with these boxes either!

If there is anything about the RF Demodulator / Modulation Monitor that may be considered a negative, it's the fact that the audio output from the box is so hi-fi that very few receivers out there will hear you like you hear yourself. To help you get a better perspective on how an average listener might hear your audio and to somewhat simulate a typical de-emphasized receiver frequency response, you may want to "de-emphasize" the output audio from the diode detector box by feeding an external EQ and bumping down the high frequencies above 1kHz using a standard 75us NRSC de-emphasis curve by adjusting your EQ with inverted 1/3 octave cut filter settings as opposed to the 1/3 octave pre-emphasis boost filters as follows:

2500 Hz (-3dB)
3150Hz (-3dB)
4000Hz (-4dB)
5000 Hz (-6.5dB)
6300Hz (-7.5dB)
8000Hz (-9dB)

The RF Sampler makes it possible to sample RF (via a BNC connector) after your amplifier for oscilloscope modulation envelope monitoring.

These two devices would be placed in your RF chain as follows in Figure 15:

Figure 15

For more on Oscilloscope setup and monitoring, see the "Scope Your Audio Modulation" page...

AM Receiver Audio Processing?

The AM receiver is as critical to a high quality AM audio experience as is the transmitter! Whether or not you will need any external EQing depends on your receiver. And even an EQ may not provide suitable results if your receiver lacks the bandwidth to effectively and accurately reproduce high-fidelity AM signals.

Your best bet is to use a receiver (or transceiver) with sufficient receiver bandwidth to start with. In most cases with modern solid state rigs, you can either make a selection on the front panel to bypass the I.F. receiver filters at will or at least change the AM receiver(s) filters with a 5kHz or more audio passband.
The older "Vintage" equipment can be a little trickier depending on what you are using.

One of the benefits of the Behringer DEQ-2496 processor mentioned in "Point 5" above is its built-in RTA (Real Time Audio Analyzer). With this device you can actually measure your receiver's audio bandwidth capabilities. There are several computer based audio spectral analysis tools available also.

It may be a good idea to find out where your receiver stands in terms of audio bandwidth and flatness. If your receiver is within about 15dB of flatness from 100Hz to 5kHz, then an EQ would prove useful in equalizing the audio. If your receiver rolls-off beyond 15dB of at least a 5kHz audio passband, you may want to consider modifying your receiver, or finding a different one if receiver accuracy is what you are looking for.

Also, receiver distortion may be an issue if it exceeds 10%. This can also be a difficult thing to remedy. All of the processing in the world cannot fix this! Just keep in mind that what you hear from your AM receiver may not be the same as what the transmitting AM stations is really transmitting! Again, the only way to know for sure is by proofing the receiver with some kind of audio spectral analyzer.

For additional reading, see: "Broadcast Chain Tutorial" and "Polycom White Paper" articles.

© 2024 NU9N - All rights reserved