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Disabling HearClear on
By Robert W. Meister WA1MIK
Motorola equipment on the 896 MHz band has noise-reduction and speech processing circuitry built into it, collectively known as HearClear. On some radios, this is split into a compandor (a compressor on the transmit audio, expander on the receive audio) and possibly something to deal with mobile flutter. Other radios just put everything into one big IC and refer to it as HearClear.
Some people like HearClear. Other people do not. Some users complain about the audio quality. Other users don't care about that. Some appreciate the reduction in noise and flutter. Others don't notice it. You could argue that Motorola wouldn't have put it in their equipment if it didn't offer some improvement, but Motorola has done stupid things in the past just like everyone else. At least they give you the ability to control HearClear on most of their mobile and portable equipment.
The MSF5000 analog (CLB) and analog-plus (GFB) repeater stations have HearClear circuitry in them too. On the CLB (EPROM-programmed) stations, small jumpers on the Station Control Board (SCB) route audio through or around the transmit compressor, the receive expander, and the Flutter-Fighter circuits individually. On the GFB (RSS-programmed) stations, signals coming from one of the big Application-Specific ICs (ASICs) on the Secure-capable Station Control Board (SSCB) route audio through or around the Flutter-Fighter and the compressor and expander (as a pair).
The compandor used in the MSF5000 offers a 2-to-1 compression or expansion ratio and has a very wide dynamic range (over 100dB). The transmit audio is compressed (a 20dB increase into the compressor results in a 10dB increase out of the compressor) while the receive audio is expanded (a 10dB increase into the expander results in a 20dB increase out of the expander). Pre-emphasis is still used on the transmitted signal and de-emphasis is used on the received signal. PL/DPL is injected after the HearClear and pre-emphasis circuits. See the data sheets at the end of this article for more detailed information.
The detected audio from the receiver passes through the HearClear expansion and flutter-fighter circuitry very early. The receive audio floating around on the station control board is normal audio. This gets sent to the MRTI connector, the headset audio output, and the TTRC. An external repeater controller will see normal audio regardless of HearClear being enabled or disabled.
Similarly, all transmit audio from the front panel MIC jack, the MRTI connector, and the TTRC is summed as normal audio and fed into the HearClear compression circuitry. After that, pre-emphasis, limiting, splatter filter, and deviation control process the audio. This gets mixed with the PL/DPL and gets sent to the transmit VCO for modulation. An external repeater controller still feeds in normal audio regardless of HearClear being enabled or disabled.
There is no field on any Radio Service Software (RSS) screen that allows the HearClear circuitry to be enabled or disabled. It's always enabled for 896 MHz stations. The SSCB has the appropriate components installed on boards destined for 896 MHz service. On all other bands, the compressor, expander, and flutter-fighter components are not present and the SSCB routes audio around them. In fact, if you could enable HearClear on an SSCB that didn't have these components, no transmit or receive audio would pass through the station.
There are other component value differences between boards, mainly dealing with audio and squelch levels that are used in wide-band (5 kHz) or narrow-band (2.5 kHz) situations. There's no need to go into these other than to mention them in passing.
Analog Station Specifics:
Analog stations use EPROMs to hold the code plug, which is programmed by the R1800/R1801 suitcase programmer. These stations are referred to as CLB stations because those three letters are the 4th through 6th characters of the model number.
The three jumpers - JU16, JU17, and JU18 - that control the expander, compressor, and flutter-fighter circuits are documented in the MSF5000 Instruction Manual and are circled in red in the board layout below. Set the jumpers depending on which of the various HearClear components you want operational. Very simple; just set it and forget it (I'm sure that's a trademarked slogan, by the way).
For completeness, here's an abbreviated list of all the user-selectable three-pin jumpers on the TLN2886A Station Control Board. There are additional soldered-in wire jumpers scattered all over the board that are used to configure other things.
|896 MHz Analog Station Control Board Jumpers|
|JU#||When in Position A:||When in Position B:|
|7||PL/DPL uses Quad Audio||PL/DPL uses External Audio|
|11||RX2 Audio to Line Out||No RX2 Audio to Line Out|
|13||PL High-Pass Filter Removed from RX||PL High-Pass Filter Inserted in RX|
|14||No IDC Control of Data||IDC Controls Data|
|16||Expander Removed from RX||Expander Inserted in RX|
|17||Compressor Removed from TX||Compressor Inserted in TX|
|18||Flutter-Fighter Inserted in RX||Flutter-Fighter Removed from RX|
|19||TX Audio Gate Controlled||TX Audio Gate Open|
|20||TX Data Routed||DC/TRC Signal Routed|
|21||IDC Controls Data||No IDC Control of Data|
Analog-Plus Station Specifics:
The analog-plus stations use a computer, RSS, RIB, and cable for programming. These stations are referred to as GFB stations because those three letters are the 4th through 6th characters of the model number.
The transmit audio compressor consists of the ICs in the following schematic. The Compandor Enable signal on U811 pin 9 is high (+9.2V) to enable the compressor. Note that this same signal feeds both the compressor and the expander. They are either both enabled or both disabled. It is not possible to enable one without the other (on the MSF5000).
The flutter-fighter consists of the ICs in the following schematic. The Flutter Fighter Enable* signal on U811 pin 10 is low (0.0V) to enable the flutter-fighter. The star (*) at the end of the signal name means it is active when low. The flutter-fighter circuit can be deactivated during the alignment procedure.
The receive audio expander consists of the ICs in the following schematic. The Compandor Enable signal on U811 pin 11 is high (+9.2V) to enable the expander. Note that this same signal feeds both the compressor and the expander. They are either both enabled or both disabled. It is not possible to enable one without the other (on the MSF5000).
On my stock 896 MHz station, the Compandor Enable signal is high (+9.2V) and the Flutter-Fighter Enable* signal is low (0.0V). For comparison, on my 450 MHz UHF station that does NOT have HearClear, the Compandor Enable signal is low (0.0V) and the Flutter-Fighter Enable* signal is high (+9.2V).
|Compandor and Flutter-Fighter Enable Signal Voltages|
|Compandor Enable||TX||Pin 9||9.2V||0.0V|
|Flutter-Fighter Enable*||RX||Pin 10||0.0V||9.2V|
|Compandor Enable||RX||Pin 11||9.2V||0.0V|
To access the signals on U811 pins 9, 10, and 11, you could remove the whole IC, install a socket, bend the three pins up, and insert the IC into the socket. A relay or other means of altering the voltages on these pins could be wired up to control the various pieces of the HearClear circuit. You can wire pins 9 and 11 together and treat them as one signal; a DPDT relay will be sufficient to connect these two lines to either 0V or +9.6V, available on U811 at pins 8 and 16 respectively. This is more work than I really care to do or even needed, and with all the CMOS components on the SSCB, the less I have to fool with it, the better.
For those who like to make circuit changes or want more details, a PDF of the data sheets for U811 and U815 are provided at the end of this article.
I'm Smarter than the Average Bear:
Only the "Minds of Motorola" know how to enable or disable these circuits. There's nothing available in the RSS screens to do it (and I'm unaware of any hidden screens there either). Or is there?
Remember earlier I mentioned that the HearClear circuitry is only present, and only enabled and utilized, on the 896 MHz band? That's a very important statement.
When you build up a code plug for the first time, one of the first fields you set is the range or band-split that the station will operate on. Naturally, we all chose 896 MHz. This setting allows receive frequencies of 896-902 MHz and transmit frequencies of 935-941 MHz. For amateur use, we setup these repeaters to receive in the 902-903 MHz range and transmit in the 927-928 MHz range. These frequencies are considered out-of-range by the RSS, but we can tell it that we know what we're doing (well, most of the time we do) and it will let us hang ourselves if we really want to.
So I decided to trick the RSS. I started with an existing 896 MHz code plug file, read it with RSS, and told RSS that I was now configuring it for an 800 MHz station. I re-entered my 896 MHz amateur-band repeater frequencies. As I expected, RSS complained but surprisingly let me continue working with the code plug. I saved this code plug file to disk and compared it to the original and found five bytes had changed, and only by a couple of bits each. I surmise some of these bits control the Compandor and Flutter-Fighter control lines in the SSCB.
I dragged my laptop computer out to the 896 MHz station, read the code plug from it, made the same change to the operating band, re-entered the TX, RX, and IDLE frequencies, and wrote the code plug back to the station. It unceremoniously accepted it and ran just fine with one exception: all of the HearClear circuitry was now disabled. Remember, it's only enabled on the 896 MHz band and the RSS just told the station that it's now an 800 MHz repeater with out-of-band (902-928 MHz) frequencies. I confirmed that the HearClear was truly disabled by measuring the various enable signals on U811; they were opposite of what they had been with the real 896 MHz code plug loaded in.
No unsoldering, no hardware modifications, no risk of breaking the plastic tabs on the front panel escutcheons while trying to remove the SSCB, no static-electricity damaging the electronics. A simple software change accomplished this in just a few minutes (I still think Motorola should have given us the ability to enable or disable HearClear). I wonder if anybody still working at Motorola knows about this back-door method? It's hard enough to find a tech support employee who knows how to spell MSF5000.
This "fix" disables both the compandor and the flutter-fighter at the same time. You can have both enabled or both disabled. There's no individual control of these two circuits. If you want the flutter-fighter activated (this is a Martha Stewart "good thing") and the compander turned off (because you don't like the sound quality), you'll have to perform a hardware modification to get that condition. These two circuits are individually controlled by the SSCB but seem to always operate as a pair, at least as far as the RSS is concerned. So, as the old adage goes, you can't always have your cake and eat it too. If you want the benefits of the flutter-fighter, you've got to take the companding along with it, at least until something better comes along.
Start the program; get past the opening screen. Connect a RIB and programming cable to the operating station. At the main menu:
When the code plug data has been successfully programmed into the station, it will reboot and come up with HearClear disabled.
Audio Observations and Adjustments:
The repeat audio seems to be louder. I now get feedback when I operate two radios close to each other. There's a lot more hiss in the received signal on radios with HearClear turned off. When both user radios have HearClear enabled, they sound fine. When both user radios have HearClear disabled, they sound fine. The CW ID from the repeater doesn't sound mushy with HearClear disabled on the repeater and the receiving radio. The station had previously been set up according to the alignment procedure in the service manual, with a maximum transmitter deviation of 2.5 kHz and the repeater level set for 1 kHz in, 1.5 kHz out, with a 1000 Hz tone.
First I fed a 1000 Hz tone at 1 kHz deviation into the receiver at -60dBm. I disabled the Receive PL using the digital metering panel. I connected a deviation meter to the output at the dummy load. The transmitter deviation measured 2.0 kHz. That is higher than I had set it a year ago. When I pushed the front panel Xmit switch, the deviation dropped just a bit to 1.95 kHz. They both should have been 1.5 kHz. For all I know, the transmit audio stages could have been limiting the deviation as well.
So I connected a 1000 Hz audio oscillator set to 1.414Vp (1.0Vrms) to TP8 and a nearby analog ground on the SSCB and keyed the transmitter with the front panel Xmit switch: the deviation went right to 1.5 kHz. This tells me that it's the repeater level that's high, not the transmitter deviation.
I read all the EEPots from the front panel display and noted their original values, then with the signal generator providing a 1 kHz deviated signal, I changed EEPot #5 (Repeater Level) so the deviation meter read 1.5 kHz. The value went from 78 to 68. I pushed the front panel Xmit switch and the deviation dropped to 1.45 kHz; close enough to 1.5 kHz for me. Adjustments complete.
Remember, the EEPot values can go from 00 to 99; your mileage may vary. Just because I lowered my value by 10 doesn't mean this same change will bring your station into alignment. You may want to adjust your system for unity gain, in which case a 1 kHz deviated input signal would provide a 1 kHz deviated output signal.
So it would appear that the MSF5000's HearClear circuitry does lower the overall repeater audio level a bit when it's enabled. If you had previously set this level with the "proper" 896 MHz configured code plug, you will need to go back and readjust this after you disable HearClear with the "trick" 800 MHz configured code plug.
I subsequently measured my GTX portable; it's putting out just over 3 kHz on the repeater input without HearClear. I was able to get the repeater to hit full deviation without even yelling into the microphone.
An Interesting Side-Effect Surfaces:
The station that I tested this modification on, was a stock New Jersey 896 MHz repeater running the firmware that came with it (4.07, 5.04), operating totally through the internal controller. As far as I could tell, it worked perfectly without HearClear enabled.
Randy N3HFK tried this on one of his repeaters that he then decided to connect to an external repeater controller. Well, things didn't work out very well. Several necessary signals mysteriously failed to operate the way they should have. After many days of testing and e-mails, he reported that he thinks he found a side-effect of making this change and a work-around.
I believe I've made an important MSF discovery. You may recall that when I initially programmed the MSF to be a full duplex base station, it wasn't passing receive audio to the external controller and I didn't have the RX1ACT light. After repeatedly playing with the programming, it started to work but I didn't know why. It was then fine for a week. Last night I was doing some programming experiments for another purpose and it stopped working again! Trying to carefully retrace my steps, I found the following:
If you want to use an external controller (full duplex base station) and don't want HearClear (changed the band-split per this article), you need to first program the MSF to be a full duplex base station, THEN do the HearClear mod and program the MSF again. If you try to do the HearClear mod first, or try to do both mods together in a single code plug write, all seems OK in the code plug but it won't work right and the RX1ACT lamp won't be illuminated.
I tried this three or four times last night and it seems to be deterministic. Your mileage may vary.
So configure the repeater as it should be (896 MHz, base station, duplex enabled), get it working with your external controller, THEN do a fresh read of the code plug, disable HearClear per this article, and write the code plug back as a last and final step. If you have to make further changes, it might be wise to re-enable HearClear during that programming session.
Acknowledgements, Credits, and Data Sheets:
Information for the analog (CLB) stations came from the MSF5000 Trunked Repeater and Repeater (RT) Stations 896-941 MHz Instruction Manual, 6881064E70, circa 1987.
Information for the analog-plus (GFB) stations came from the MSF5000 Digital-Capable and Analog Plus 900 MHz Service Manual, 6881092E90, circa 1994.
Rick K1RJZ started me thinking about this when he asked if it was possible to disable HearClear on the New Jersey MSF5000 repeaters via hardware or software.
Dennis WB6OIL confirmed most of the information contained herein.
Mike WA6ILQ gave me some additional ideas and suggestions and proofread the article.
Randy N3HFK discovered one side-effect of doing this modification and was kind enough to allow me to share his experience with the rest of the readers.
MSF5000, HearClear, RSS, RIB, GTX, and a whole bunch of other terms are trademarked by Motorola, Inc.
A data sheet for the three-section analog multiplexer (switch), U811, marked MC14053BCP, can be found here.
A data sheet for the two-section compandor (compressor/expander, U815, marked NE570, can be found here.
A similar data sheet for an alternate part, SA571, can be found here.
The author can be contacted at: his-callsign [ at ] comcast [ dot ] net
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This page originally posted on Monday 30-Mar-2009
Article text, artistic layout, and hand-coded HTML © Copyright 2009 by Robert W. Meister WA1MIK.
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.