Date sent: Mon, 26 Mar 2001 21:53:04 -0600 From: "George, W5YR" To: icom@qth.net, PSK31 List , heilsound@qth.net Subject: [Icom] Icom IC-756PRO Audio Response Functions (long) Send reply to: icom@qth.net From: "George, W5YR" Even before I purchased my PRO in September of last year, I had heard glowing reports of its superior transmit and receive audio. Initially, I put the reports to the usual enthusiasm for a new product. But, after setting up my own PRO and using it a few days, I became convinced that the little box did, indeed, provide audio performance a cut above the usual amateur radio transmitters and receivers that I had been exposed to in over 55 years in the hobby. After using the PRO for a few months, I was persuaded to extend my audio horizons by using the rear-panel ACC(1) connector to input transmit audio and output received audio. The motivation for this approach was two-fold: to avoid the frequency-range and dynamic-range and distortion limitations of the microphone analog input channel, and to extract received audio before subjecting it to the, again, limited frequency-range and dynamic-range analog audio circuitry of the PRO. I have been very pleased with the results. While I am not a big fan of "hi-fi" audio on SSB, I do appreciate having a clean transmitted signal whose frequency characteristics can be configured to fit the requirements of ragchewing or a DX pileup. I have been most impressed, however, by the enormous improvement in receive audio quality that I have obtained by routing the audio output through a modest speaker amplifier and speaker system. Most SSB signals are now much more "readable" and clearly more pleasant to listen to. Many can be made much easier to read by appropriately tailoring the receive audio frequency response. The key to this approach has been the Behringer MX602 Mixer/Equalizer, an inexpensive (under $100) professional audio product readily available in most large music stores or from Heil Sound. Details can be obtained at http://www.behringer.com but, briefly, it is a basic four-channel mixer with mic pre-amps, equalization controls, gain controls, etc. I use it to interface the PRO transmit audio input, the PRO receive audio output, and the computer soundcard mic input and speaker output. I also use it for equalized preamps for my Heil Goldline and ProSet HC5 mics. Another output drives one channel of an oscilloscope for continuous time-domain monitoring of the all transmitted and received signals. A complete operating manual can be downloaded from the URL above. The 602 has placed total audio control of the entire station in one unit, making it possible to set mic gains, volume levels, equalization, etc. all from one convenient location. The 602 is a highly professional audio product with imperceptible hum and noise levels. Thus far, it has been impervious to r-f despite the main antenna passing directly over the shack at the 30 ft level. After installing the 602 and beginning to enjoy its performance, I became curious as to the frequency range I was able to hear from the PRO receiver and the range I would be able to input to the transmit section. So, I devised some simple measurements to determine (a) the transmit audio power response for the three available transmit bandwidths (NAR, MID, and WIDE) and (b) the receive audio frequency response; and to estimate the single-tone distortion generated in the transmit audio section when operating PSK31. Following are the results of those measurements; please use a non-proportional font for viewing: TRANSMITTER AUDIO POWER RESPONSE Mode Frequency Range (Hz) Bandwidth (Hz) Response (db) WIDE 87 - 2911 2824 -3 75 - 2924 2849 -6 62 - 2940 2878 -10 MID 295 - 2703 2408 -3 283 - 2713 2430 -6 267 - 2728 2461 -10 NAR 493 - 2700 2207 -3 481 - 2711 2230 -6 466 - 2725 2259 -10 NAR 445 - 2744 2299 -3 COMP 431 - 2758 2327 -6 410 - 2777 2397 -10 The setup for these measurements consisted of a computer-generated constant-amplitude sinewave signal interfaced via the MX602 (no equalization) to the ACC(1) TX audio input. Output level was read from the panel meter at the 100% (0 db), 50% (-3 db), 25% (-6 db), and 10% (-10 db) points. Data were taken at 14,200 KHz with a 100% power output of 100 watts at 1000 Hz audio input. It is unprofessional to present data whose apparent precision overstates their underlying accuracy, but rather than take time to round things off, I have just listed the data I took in the shack. Please don't try to reproduce these exact values with your PRO! <:} You will note that the WIDE and MID settings produce transmit bandwidths centered at 1500 Hz, as one would expect from the DSP filter structure of the PRO. (Lower limit + 1/2-bandwidth ~= 1500 Hz). This is not the situation with the NAR setting however, since the data suggests a center frequency closer to 1595 Hz. Turning on 9 db of compression in NAR mode, to simulate a typical DX-pileup configuration, produces a slightly wider bandwidth but no other effect was noticed. TRANSMIT SINGLE-TONE AUDIO DISTORTION Input Frequency (Hz) Output Level Spectral Response (db) 500 50% 500 Hz 0 1000 -41 1500 -57 1000 50% 1000 Hz 0 2000 -39 3000 -60 1500 50% 1500 Hz 0 3000 -62 Lacking an r-f spectrum analyzer to measure distortion products directly, I opted to estimate the likely distortion by looking instead at the audio spectrum of the MONITOR output of the PRO under single-tone excitation. Since the MONITOR output is a faithful replica of the audio signal ultimately translated to r-f, amplified and output, it should serve as a useful indicator of the distortion introduced by the DSP audio section, etc. under the conditions normally encountered with PSK31 and other audio-tone modes. For these measurements, the sinewave output of an HP 200C oscillator was fed through the MX602 as for the bandwidth tests. The MONITOR output of the PRO was fed directly into the soundcard to the SpectroGram program for spectral analysis. Operation was at the 50% or 50 watt power level, typical for PSK31 operation, and WIDE mode with no compression or ALC activity. Based on these results, one should be able to operate with an audio frequency as low as 500 Hz without undue concern over harmonic generation in the PRO. Nonetheless, conventional wisdom among PSK31 operators is to use an audio frequency at or above 1000 Hz to minimize potential distortion. Clearly, operation at 1500 Hz and above should avoid any prospect of harmonic distortion. RECEIVE AUDIO RESPONSE Frequency Range (Hz) Bandwidth (Hz) Response (db) 100 - 3450 3350 -3 74 - 3461 3387 -6 45 - 3496 3451 -10 These data were obtained by detuning the internal calibrator signal at 50,100 KHz in USB mode. PRO audio output was fed to the SpectroGram program via the MX602 (no EQ). DSP IF bandwidth was the default 3.6 KHz (FIL1). These results confirm the comparatively wide frequency response of the PRO receive audio when it is taken very nearly directly from the DSP data stream. With the Heil Proset, I can clearly hear 20-30 Hz signals from the PRO, although my speaker system drops off below about 40 Hz. I can see response below 10 Hz on the monitor scope. Such extended bass response reminds one of "the good old AM days." Lest anyone conclude that I advocate extreme frequency bandwidth for amateur radio SSB, I am fully aware of the power waste of excessive low-frequency content in transmitted signals, as well as the undesirability of excessive high-frequency content leading to broad signals. But, it is informative to understand the capabilities of one's equipment, and to have the convenience of a unit such as the MX602 to tailor the frequency performance at will to suit operating needs. It would be instructive for someone to make similar measurements using the conventional front-panel mic input and speaker output connections. I apologize for the length of this posting, but I recall reading several requests for "someone to sweep the PRO" and find out what frequency ranges it actually provides. I hope that these data will offer some response to those requests. 72/73, George W5YR - the Yellow Rose of Texas QRP-L 1373 NETXQRP 6 Fairview, TX 30 mi NE of Dallas in Collin county EM13qe Amateur Radio W5YR, in the 55th year and it just keeps getting better! Icom IC-756PRO #02121 Kachina #91900556 IC-765 #02437 ---- Your Moderator: Dick Flanagan W6OLD, owner-icom@qth.net List Archives : http://www.qth.net/archive/icom/icom.html To unsubscribe: