Running AM with the IC-756 Pro III
There are quite a few people
running the 756 Pro series of rigs, who have discovered that it has
a very nice AM receiver. The available bandwidths include 3 kHz, 6
kHz, and 9 kHz positions, with the two
wider ones being capable of providing some very pleasant listening
experiences, especially when a good quality speaker or audio system is
connected. Unfortunately, the AM transmit side of the stock 756 Pro
series is a somewhat different story. Even with a high quality, flat
response microphone, the stock internal audio produces a very
restricted sounding
signal, with no processing or equalization. There is a bit of deception
going on here as
well, with the built-in monitor of the
756 Pro series. When you listen
to yourself in the monitor with a pair of
headphones, your transmitted signal actually sounds pretty
decent
on AM. Don't be fooled! I believe they
are picking off the signal before the DSP and ALC circuits become
involved in the process. Due to the hard coded DSP settings, there are
some limitations as far as what you can do when transmitting AM with
the Pro series, but it's possible to make it sound fairly presentable
by avoiding the internal transmit audio and running clean low level
audio
into the back panel input.
When I contemplated using external audio, it dawned on me that I already had an audio chain in place, through the digital mode interface between my station computer's sound card and the Pro III. The interface that I use is a SignaLink SL-1+ but anything that offers a decent audio transformer to isolate the sound card and the Pro III would work just fine. The transformer in the SignaLink SL-1+ is a 600 ohm to 600 ohm transformer that looks very much like what was used on old telephone modem cards. The setup is very simple as illustrated in the picture below.
I grabbed one of my 5 dollar, home-made, flat response condenser microphones, and made a shielded cable with an XLR connector on one end and a 1/8 inch stereo plug on the other to allow connection to the microphone input of a sound card. It sounded very nice with headphones plugged directly into the computer, however, when I tested the audio through the Pro III it sounded very boomy and muffled. Obviously, some audio processing was going to be necessary. A search of readily available programs brought me to an amazing piece of sofware called Voice Shaper.
Voice Shaper Audio Processing Software
This amazing little program, written by Alex, VE3NEA, is ME/2000/XP compatible and allows 7 points of EQ adjustment between 70 Hz and 6000 Hz. It also offers a compressor, limiter, noise gate, and a brick wall audio bandpass filter as well. Another nice feature is that you can save as many different presets as you like. The nicest surprise is that this amazing little program is free! The learning curve with this software is very reasonable, so with a little experimentation you can produce many different audio profiles. I won't get into the finer points of using this program since the author covers this in great detail with an online tutorial. The best thing to do is to play around with all the features and see for yourself how it works. The screenshot below shows one preset that I use on AM when conditions are good. The compression is set at 1.2:1 and the Output Gain is running at -12 dB.
When I contemplated using external audio, it dawned on me that I already had an audio chain in place, through the digital mode interface between my station computer's sound card and the Pro III. The interface that I use is a SignaLink SL-1+ but anything that offers a decent audio transformer to isolate the sound card and the Pro III would work just fine. The transformer in the SignaLink SL-1+ is a 600 ohm to 600 ohm transformer that looks very much like what was used on old telephone modem cards. The setup is very simple as illustrated in the picture below.
I grabbed one of my 5 dollar, home-made, flat response condenser microphones, and made a shielded cable with an XLR connector on one end and a 1/8 inch stereo plug on the other to allow connection to the microphone input of a sound card. It sounded very nice with headphones plugged directly into the computer, however, when I tested the audio through the Pro III it sounded very boomy and muffled. Obviously, some audio processing was going to be necessary. A search of readily available programs brought me to an amazing piece of sofware called Voice Shaper.
Voice Shaper Audio Processing Software
This amazing little program, written by Alex, VE3NEA, is ME/2000/XP compatible and allows 7 points of EQ adjustment between 70 Hz and 6000 Hz. It also offers a compressor, limiter, noise gate, and a brick wall audio bandpass filter as well. Another nice feature is that you can save as many different presets as you like. The nicest surprise is that this amazing little program is free! The learning curve with this software is very reasonable, so with a little experimentation you can produce many different audio profiles. I won't get into the finer points of using this program since the author covers this in great detail with an online tutorial. The best thing to do is to play around with all the features and see for yourself how it works. The screenshot below shows one preset that I use on AM when conditions are good. The compression is set at 1.2:1 and the Output Gain is running at -12 dB.
The real truth is in what is
actually heard with the Pro III and external sound card audio
processing. Using the EQ curve in the screenshot above, and a
compression setting of 1.2:1 I recorded a few mp3 files of the Pro III
in action. The two sound clips derived directly from the Pro III's
headphone jack tell the story about how deceiving the built-in monitor
of the Pro III can be. What you hear through the Pro III monitor is NOT
what people hear on their receivers during an actual QSO!
Pro III with stock audio on AM as it sounds through the Pro III monitor size is 210 kb - mp3 What people think they sound like with the stock Pro III audio. This audio sample was made by patching the headphone jack monitor output of the Pro III directly into the line input of one of my soundcards.
Pro III with stock audio on AM as heard by an FT897D with a 5 kc AM filter size is 320 kb - mp3 What it actually sounds like to someone else over the air. Fairly restricted with a midrange peak and the characteristic sound of a carbon microphone from a an old telephone.
The three audio clips below are samples of the Pro III with Voice Shaper processed audio injected into the back port.
Pro III with Voice Shaper audio on AM as it sounds through the Pro III monitor size is 340 kb - mp3 Wouldn't it be nice if it actually could sound like this? This audio sample was made by patching the headphone jack monitor output of the Pro III directly into the line input of one of my soundcards. Obviously, the monitor tap in the Pro III picks off the sample before the DSP and ALC do their work.
Pro III with Voice Shaper audio on AM as heard by an FT897D with a 5 kc AM filter size is 320 kb - mp3 Certainly not perfect, but smoother than the stock audio.
Pro III with Voice Shaper audio on AM as heard by a Yaesu FRG7700 with its 12 kc AM filter size is 486 kb - mp3 This is a best case receive scenario, and represents what is heard with a receiver designed for AM reception. It sounds much better than what is heard through most sideband receivers where they tend to use a narrow filter in the AM mode. You can still hear things happening on some sibilant peaks, but the audio is generally smooth and not too hard on the ears. Note that the tone control on the 7700 was rolling off the high end somewhat in this recording.
Several people have asked me how far you can push the low-end and high-end response with the Pro III. I messed around with a preset that boosted the low end response at 70 Hz to +18 db and pushed the high end response at 5700 Hz to +16 db. An attempt was made to create quite a bit of presence rise as well by pushing the area around 3kc up to +10 db. The Icom's DSP will certainly cut into those settings, but the shape of the audio response is definitely affected. Additionally, I ran the compression up to a level of 2:1 which is probably most noticable by the increase in background noise between syllables. The end result is not all that bad, but you can judge it for yourself by listening to the mp3 link underneath the Voice Shaper pictures showing how the EQ and compression are set up.
Pro III with Voice Shaper compression at 2:1 and both the low-end and high-end set near the max size is 210 kb - mp3 Surprisingly, even with the edges being pushed, it still stays pretty clean. Most of the high frequency energy that is heard is coming from the peak at 3 kc but the over-all effect makes it sound much brighter. I can hear some crunching evident on sibilant sounds, but it probably would not be too apparent on the other end of a QSO. Note that the tone control on the FRG-7700 receiver was set at roughly the middle or neutral position. There might be a bit more low end energy present, but it's hard to tell. The brick wall audio filter in Voice Shaper does a great job of stopping speech frequencies that are way beyond what the rig's DSP will tolerate. I believe this prevents the Icom's DSP from producing a lot of artifacts, which triggers a response from the ALC. This is a big problem that people run into when they use one of the cheap 3 band type mic preamp/equalizers such as the little Behringer 802. That third EQ band is totally outside the range of the Icom's DSP, and if you attempt to crank it up, the DSP and ALC appear to respond in an unfortunate way.
The audio clip below was sent to me by a friend who has a very nice Flex Radio system. In addition to sounding beautiful in any mode, the Flex makes very clean recordings of signals as they are heard over the air. In this clip of a 75 meter QSO, two other AM stations can be heard along with the Pro III, which is using Voice Shaper and being amplified by an AL-80B running about 175 watts of AM carrier. My voice on the Pro III is the third voice that you hear in the recording.
Pro III during a QSO on 75 meters size is 210 kb - mp3 Again, it's certainly not perfect, but the Pro III can be made to sound pretty good, in my opinion.
Even though I worked carefully to avoid slamming the audio into the DSP of the Pro III, you can still hear some artifacts in the audio, especially on sibilant peaks. I believe this is inevitable given the parameters that are hard coded into the Pro III firmware. If you try to ram lots of high frequencies way outside the range of the DSP, the audio becomes very gritty and unpleasant to those who have to listen to it. I believe the trick is to insert some presence rise near 3000 Hz to avoid sounding muffled. Under rougher conditions, simply make a profile that rolls off the low end and then increase the Voice Shaper compression level and boost the slider near 3000 Hz to brighten the audio response. Again, don't rely on the Pro III monitor when making audio level adjustments, it simply does not accurately tell you what is happening. Depending on the capabilities of your computer, you will experience some degree of latency that will be noticable if you monitor yourself. The slower the computer is, the more latency you will experience. It was quite noticeable (more than half a second) when I used an older system with a 2.4 GHz processor and improved drastically when I installed my recently retired gaming system that uses a 3.0 GHz hyperthreading processor. Annoying audio dropouts may also occur with a slower computer, so keep this in mind if you intend to use an older system.
When setting up your transmit audio it is best to use an easily built RF sampling monitor such as the one shown in the diagram below. This circuit is based upon a design from the East Coast Sound section of the AMfone archives. The original design works fine for power levels up to about 30 watts, but it tends to emit smoke at higher power levels! I run the audio output of the detector below into a control box that switches my headphones between the receiver output and the AM Diode Detector output whenever I key the rig. If you use headphones that have an inline volume control you can easily balance the audio level, but if you use studio headphones it would be advisable to add a variable resistor at the output of the AM detector circuit to allow you to set the audio output to a comfortable level. There is a huge repository of excellent AM information at the amfone.net site!
As an alternative, you can use a receiver and plug some headphones in to allow you to make level adjustments through your soundcard software and the Voice Shaper software. Keep in mind that there may be a bit of overload induced blocking going on when using a second receiver, but it will give a pretty good indication of what your audio sounds like. I found that it was critical to watch the sound card input and output levels to prevent getting into a situation where distortion was introduced before audio was injected into the Pro III. A very helpful utility called QuickMix can be used to save your Windows mixer settings to a file when you find settings that are acceptable. When other programs alter the settings you can load your audio profile back into the Windows mixer with the QuickMix utility. I have three sound cards in my station computer, so it was essential to use QuickMix to keep track of the settings for all three of them! The sound card I use for producing audio for the Pro III is a cheap Sound Blaster Live PCI card. QuickMix is a free program that can be downloaded here:
QuickMix Soundcard Settings Utility
I found input and output sound card level settings that produced clean audio which then allowed me to use the "Output Gain" slider in Voice Shaper to control the audio level on the fly. This setting is calibrated in dB and makes it simple to fine tune your settings. This setting is also "remembered" when you save each Voice Shaper profile onto your hard drive. After a lot of help from others over the air, I found with my sound card configuration that a setting of -12 dB produced a fully modulated signal without sounding overdriven. Your own settings will vary from mine, but you will find a setup that will work with your equipment. I pretty much leave all the levels alone now, and have made up 5 or 6 different Voice Shaper profiles that can be switched on-the-fly to accomodate conditions and the receiver being used on the other end.
If you have an interest in using your IC-756 Pro series transceiver on AM, you might want to give this simple audio processing method a try. If you already have a computer and digital interface, you won't need to invest in a pile of hardware to get decent results. One other important factor to consider with the Pro III is the power level it is used at in the AM mode. I have found that if you climb much above 20 watts output, that things begin to fall apart rather quickly. Typically, I run around 15 watts into my AL-80B 3-500 amp and get 150 watts of carrier output. With an average reading wattmeter I can see the meter deflect upwards slightly on audio peaks. As expected, a peak reading wattmeter will show major positive swings as you modulate. Also, keep in mind that you can use the same external audio chain to experiment with other modes as well.
Pro III with stock audio on AM as it sounds through the Pro III monitor size is 210 kb - mp3 What people think they sound like with the stock Pro III audio. This audio sample was made by patching the headphone jack monitor output of the Pro III directly into the line input of one of my soundcards.
Pro III with stock audio on AM as heard by an FT897D with a 5 kc AM filter size is 320 kb - mp3 What it actually sounds like to someone else over the air. Fairly restricted with a midrange peak and the characteristic sound of a carbon microphone from a an old telephone.
The three audio clips below are samples of the Pro III with Voice Shaper processed audio injected into the back port.
Pro III with Voice Shaper audio on AM as it sounds through the Pro III monitor size is 340 kb - mp3 Wouldn't it be nice if it actually could sound like this? This audio sample was made by patching the headphone jack monitor output of the Pro III directly into the line input of one of my soundcards. Obviously, the monitor tap in the Pro III picks off the sample before the DSP and ALC do their work.
Pro III with Voice Shaper audio on AM as heard by an FT897D with a 5 kc AM filter size is 320 kb - mp3 Certainly not perfect, but smoother than the stock audio.
Pro III with Voice Shaper audio on AM as heard by a Yaesu FRG7700 with its 12 kc AM filter size is 486 kb - mp3 This is a best case receive scenario, and represents what is heard with a receiver designed for AM reception. It sounds much better than what is heard through most sideband receivers where they tend to use a narrow filter in the AM mode. You can still hear things happening on some sibilant peaks, but the audio is generally smooth and not too hard on the ears. Note that the tone control on the 7700 was rolling off the high end somewhat in this recording.
Several people have asked me how far you can push the low-end and high-end response with the Pro III. I messed around with a preset that boosted the low end response at 70 Hz to +18 db and pushed the high end response at 5700 Hz to +16 db. An attempt was made to create quite a bit of presence rise as well by pushing the area around 3kc up to +10 db. The Icom's DSP will certainly cut into those settings, but the shape of the audio response is definitely affected. Additionally, I ran the compression up to a level of 2:1 which is probably most noticable by the increase in background noise between syllables. The end result is not all that bad, but you can judge it for yourself by listening to the mp3 link underneath the Voice Shaper pictures showing how the EQ and compression are set up.
Pro III with Voice Shaper compression at 2:1 and both the low-end and high-end set near the max size is 210 kb - mp3 Surprisingly, even with the edges being pushed, it still stays pretty clean. Most of the high frequency energy that is heard is coming from the peak at 3 kc but the over-all effect makes it sound much brighter. I can hear some crunching evident on sibilant sounds, but it probably would not be too apparent on the other end of a QSO. Note that the tone control on the FRG-7700 receiver was set at roughly the middle or neutral position. There might be a bit more low end energy present, but it's hard to tell. The brick wall audio filter in Voice Shaper does a great job of stopping speech frequencies that are way beyond what the rig's DSP will tolerate. I believe this prevents the Icom's DSP from producing a lot of artifacts, which triggers a response from the ALC. This is a big problem that people run into when they use one of the cheap 3 band type mic preamp/equalizers such as the little Behringer 802. That third EQ band is totally outside the range of the Icom's DSP, and if you attempt to crank it up, the DSP and ALC appear to respond in an unfortunate way.
The audio clip below was sent to me by a friend who has a very nice Flex Radio system. In addition to sounding beautiful in any mode, the Flex makes very clean recordings of signals as they are heard over the air. In this clip of a 75 meter QSO, two other AM stations can be heard along with the Pro III, which is using Voice Shaper and being amplified by an AL-80B running about 175 watts of AM carrier. My voice on the Pro III is the third voice that you hear in the recording.
Pro III during a QSO on 75 meters size is 210 kb - mp3 Again, it's certainly not perfect, but the Pro III can be made to sound pretty good, in my opinion.
Even though I worked carefully to avoid slamming the audio into the DSP of the Pro III, you can still hear some artifacts in the audio, especially on sibilant peaks. I believe this is inevitable given the parameters that are hard coded into the Pro III firmware. If you try to ram lots of high frequencies way outside the range of the DSP, the audio becomes very gritty and unpleasant to those who have to listen to it. I believe the trick is to insert some presence rise near 3000 Hz to avoid sounding muffled. Under rougher conditions, simply make a profile that rolls off the low end and then increase the Voice Shaper compression level and boost the slider near 3000 Hz to brighten the audio response. Again, don't rely on the Pro III monitor when making audio level adjustments, it simply does not accurately tell you what is happening. Depending on the capabilities of your computer, you will experience some degree of latency that will be noticable if you monitor yourself. The slower the computer is, the more latency you will experience. It was quite noticeable (more than half a second) when I used an older system with a 2.4 GHz processor and improved drastically when I installed my recently retired gaming system that uses a 3.0 GHz hyperthreading processor. Annoying audio dropouts may also occur with a slower computer, so keep this in mind if you intend to use an older system.
When setting up your transmit audio it is best to use an easily built RF sampling monitor such as the one shown in the diagram below. This circuit is based upon a design from the East Coast Sound section of the AMfone archives. The original design works fine for power levels up to about 30 watts, but it tends to emit smoke at higher power levels! I run the audio output of the detector below into a control box that switches my headphones between the receiver output and the AM Diode Detector output whenever I key the rig. If you use headphones that have an inline volume control you can easily balance the audio level, but if you use studio headphones it would be advisable to add a variable resistor at the output of the AM detector circuit to allow you to set the audio output to a comfortable level. There is a huge repository of excellent AM information at the amfone.net site!
As an alternative, you can use a receiver and plug some headphones in to allow you to make level adjustments through your soundcard software and the Voice Shaper software. Keep in mind that there may be a bit of overload induced blocking going on when using a second receiver, but it will give a pretty good indication of what your audio sounds like. I found that it was critical to watch the sound card input and output levels to prevent getting into a situation where distortion was introduced before audio was injected into the Pro III. A very helpful utility called QuickMix can be used to save your Windows mixer settings to a file when you find settings that are acceptable. When other programs alter the settings you can load your audio profile back into the Windows mixer with the QuickMix utility. I have three sound cards in my station computer, so it was essential to use QuickMix to keep track of the settings for all three of them! The sound card I use for producing audio for the Pro III is a cheap Sound Blaster Live PCI card. QuickMix is a free program that can be downloaded here:
QuickMix Soundcard Settings Utility
I found input and output sound card level settings that produced clean audio which then allowed me to use the "Output Gain" slider in Voice Shaper to control the audio level on the fly. This setting is calibrated in dB and makes it simple to fine tune your settings. This setting is also "remembered" when you save each Voice Shaper profile onto your hard drive. After a lot of help from others over the air, I found with my sound card configuration that a setting of -12 dB produced a fully modulated signal without sounding overdriven. Your own settings will vary from mine, but you will find a setup that will work with your equipment. I pretty much leave all the levels alone now, and have made up 5 or 6 different Voice Shaper profiles that can be switched on-the-fly to accomodate conditions and the receiver being used on the other end.
If you have an interest in using your IC-756 Pro series transceiver on AM, you might want to give this simple audio processing method a try. If you already have a computer and digital interface, you won't need to invest in a pile of hardware to get decent results. One other important factor to consider with the Pro III is the power level it is used at in the AM mode. I have found that if you climb much above 20 watts output, that things begin to fall apart rather quickly. Typically, I run around 15 watts into my AL-80B 3-500 amp and get 150 watts of carrier output. With an average reading wattmeter I can see the meter deflect upwards slightly on audio peaks. As expected, a peak reading wattmeter will show major positive swings as you modulate. Also, keep in mind that you can use the same external audio chain to experiment with other modes as well.