AUDIO Level settings for remote radio tone control circuits

[Jim202]

This will probably open up a hornets nest of sores from some people, but I have to ask.

In my travels around the country, I have noticed that no two people, let alone different radio
shops seem to set up the audio levels on a tone remote telephone or microwave link the same
way. To elaborate some on this, let me start out by saying there is a standard for the audio
control tones used in a tone remote controlled radio. If you do some home work on the subject,
you will find that the wakeup tone should be set at a level +10 db above the voice level. This
is a 120 millisecond burst of a 2175 Hz tone. Following the first burst is the "FUNCTION" tone
that is about 40 milliseconds long, at a 0 db level with the voice. The frequency of this tone
will be determined as to what function is being sent. A normal transmit F1 function would be
a tone of 1950 Hz. Selection of the F2 would be a tone of 1850 Hz and so on. Following the
"FUNCTION" tone is the 2175 Hz "LOW LEVEL GUARD" tone that keeps the transmitter keyed up.
This level normally is at a level of -20 db below the voice level.

So here is the big question, just what do you use as the "standard VOICE LEVEL" for these
adjustments? Many of the Motorola shops seem to set the "HIGH LEVEL WAKEUP" tone to
a -10 db setting. I don't have a problem with this level. It works out real well for most
locations. The problem comes up when you have a telephone company that provides a
4 wire circuit with a built in -10 db loss. If you start out with a -10 db for the "WAKEUP" tone,
the phone company adds another -10 db loss, you end up with the idle tone level of a
-40 db at the radio.

As most good radios using a built in tone control stop working at a -40 db "LOW LEVEL IDLE"
tone, you can see that your at the bottom of the operating range of the control system
without any additional line losses.

The question before us is how are you setting these levels to make sure that you have a built
in buffer of the control tone levels to insure reliable remote radio operation? In asking how well
the radios work at some of these 911 dispatch centers, I am getting the message that they
are having problems at times in keying up the remote radios. In some cases, they key up the
radio, but it will randomly drop out while the dispatcher is talking.

I have been using a storage scope to look at the levels these circuits are adjusted to. The
reason being is that unless your using a Motorola BIM card to originate the tones with, there
is no easy way for a dispatch operator to key a tone circuit and you be able to measure the
tone levels. Sure you can measure the "LOW LEVEL IDLE" tone level with a TIMS, but that
doesn't show you at what levels the other two burst tones are at.

What makes this whole topic even harder to comprehend, is that I have seen a radio tech
just walk up to Motorola CEB rack, pull out a screw driver and twist the level setting pot
on a BIM card and say "That's where the level should be". As all of us know, the level setting
pot on a BIM card is about as course of adjustment you can find. It just boggles my mind that
Motorola ever let that out of the design engineering this way. My point is, without some sort
of test equipment, you can't just walk up to a level setting pot and crank on it. They say that
is the level it should be at. The BIM adjustment is like if you even breath on it while making
the adjustment, it will change by several db. Hardly a good way of making level adjustments.

Comments anyone?

Jim

[wavetar]

The "standard" voice level target, in the Motorola world at least, is -10db. This would put the HLGT at 0db, and the LLGT at -30db. Most Motorola remotes & TRA's that I've dealt with coming out of the box are set to this level at the factory. So, if I'm setting up equipment 'locally', ie: with direct copper wire within the same building for instance, then the level stays put in both sets of equipment. If I'm locating equipment remotely through leased lines with a defined loss of -10 or -16db, then I adjust both sets of equipment accordingly. All the information required to do this is in the appropriate installation manuals. Ultimately, until I check the levels from both ends & confirm it's what it should be, the job isn't finished.

The tech that is setting the BIM judging by physical indication of the BIM pot, is an idiot. BIM's are extremely easy to set the level...whenever you push the reset button the BIM automatically sends out 3 sets of tones, several seconds each. A low, medium & high tone frequency. Put your tested on the output & measure. Reset & adjust (generally for the medium tone) until the desired reading is achieved.

Todd

[GlennD]

If you set for +10 db in the digital world you will get very unreliable keying since the mux will not have enough bits and it will strip the hi guard tone wake up. We use a max of 0Bd.

With rare exceptions all of our traffic is carried on our in house microwave links and we have had a tech set the level too high. This caused a very intermittent keying problem.

[wavetar]

If you set for +10 db in the digital world you will get very unreliable keying since the mux will not have enough bits and it will strip the hi guard tone wake up. We use a max of 0Bd.

With rare exceptions all of our traffic is carried on our in house microwave links and we have had a tech set the level too high. This caused a very intermittent keying problem.

Yes, you do have to work within limitations of the linking equipment. Leased lines have no issues with a HLGT of +10db & subsequent voice level of 0db & LLGT of -20db. It's quite standard, actually. I have limited experience with microwave links, but the levels allowed depend on the level of loss set.

[Jim202]

All good comments so far.

I didn't want to come right out and give my opinion of the tech doing the visual adjustment on the BIM output.
The funny part about him was he was so concerned about the paging tone levels being effected. Never did
see him with a meter to measure the actual levels.

As with all microwave systems, you can't exceed the input level limit and expect it to function normally.

Ran into an MCC5500 system that the high level wakeup tone was over 10 volts peak to peak as measured on
my storage scope. As about 5.6 to 5.8 volts is 0 db, my gut feeling is the radio tech that set up that circuit
didn't measure it either. Most of the circuits I have been looking at recently all seem to fall in the 4.5 to 5.5
volts peak to peak level on the wakeup tone.

Have you ever found a tech that reads the installation manual?

Jim

[wavetar]

Have you ever found a tech that reads the installation manual?

Jim

Other than myself? Precious few! It's amazing what one can learn when one reads them...it's almost like they're a collection of information dealing exclusively with the product they came with...

[SlimBob]

Send at +6, recieve at -10.

[RFguy]

I agree with Wavetar here. We use -10 dBm for voice (so 0 dBm for HLGT) for in-building and 0 dBm for voice, (so +10 dBm for HLGT) for lease lines.

I would consider microwave where there are no lease lines involved to be the same as in-building.

Also mentioned above was a BIM and paging tones. Note that Motorola does paging tones properly, they de-emphasis them. So a 1000 Hz paging tone set for 0 dBm will be +11 dBm at 288 Hz. +11 is the max allowable on lease lines to avoid cross talk. So if you are doing paging, don’t go over 0 dBm for your 1000 Hz tone.

I also use a storage scope (my handy Fluke 123) to "capture" the tome sequence and look for the 3 levels of tones at the proper level.

[Tom in D.C.]

Old ham radio axiom, still true even today:

"When all else fails, read the instructions!"

[Jim202]

That would be great if the other radio tech's did it. However when you walk in cold and are trying to interface
to a circuit that some yahoo set up improperly, things can get interesting real fast. The normal comment is
"It was working before you got here, it must be something wrong with the new equipment". Makes it tough
to follow an act like this.

Jim

Old ham radio axiom, still true even today:

"When all else fails, read the instructions!"

[kb4mdz]

Good, but another trouble is every set of directions is different from manufacturer to manufacturer, and, from my readings, there are 'unstated assumptions' about certain aspects. Ever look carefully at the instructions for the simple, everyday CPI TTP1 Tone Remote Adapter? It lists "Line Input Level (from phone lines) as -40 to +10 dBm (Function Tone Referenced)" Uhhh, what EXACTLY does that mean?

Farther on in the installation instructions, it calls for 'generate a constant function tone, which ideally should be 0 dBm at the remote site'. Then adjust R64 for 460mVrms, 1.3Vpp. Or 46 mVrms (130mVpp) if you're using LLGT to set the test point TP1

See, they're referencing to 0 dBm Function Tone, which is 'supposed' to be about average voice level. Right?

OK, lets say you're starting FT at dispatch at 0 dBm, and line loss means you're getting FT at -12 dBm at the remote site.

And what is equivalent, if your FT starts at the originating end as -10 dBm? On the face of it, that would put FT at -22 dBm at the station, thus LLGT 20dB below that, at -42 dBm, at the station, wouldn't it? Is that getting too low for the TRA to decode 2175 Hz, and properly generate PTT?

Do they really imply that no matter what line loss, 0dB or 16 dB, you set TP1 for 460 mVrms?

Really, then you have to make sure the instructions from different equipments will make a workable, usable system

A usable system defined as "Shall not rust, bust, or collect dust." ; it does what is intended, doesn't break under normal use, or sit in a closet because it's too complex or doesn't do what's needed.

Inquiring minds want to know,

Old ham radio axiom, still true even today:

"When all else fails, read the instructions!"

[wa6jbd]

Good topic, and one dear to my heart...

A lot is riding on the transmission medium between the BIM and the radio. It would be nice to assume that THAT part of the circuit is adjusted correctly, but it isn't always so. In a digital MUX, there is about 3 db of headroom between 0TLP, and running out of bits. Where it hits the codec on the card, it's really closer to -16 (or -17.5 on some), so some pad is inserted on 4WTX and 4WRX to get a circuit that's 0-0 across the span. Assuming this part is done right, that circuit has an absolute maximum power level of 0 dbm that can be applied - and this likely is true for phone lines, since most are now carried on T1 or some similar carrier. A pure copper wireline doesn't care nearly so much. And if you're running 16 db loss on your phone lines, you ordered a data circuit, not a radio circuit.

So... All that means is that a +10 DBM high guard tone level is TOO HIGH to be transported. I believe THAT old standard dates back to the use of dry copper telco pairs.

As to the rest of the level standards, generally, I've seen -10 dbm set to equal 2/3 system deviation, continuous tones such as status tone to -13, and data at -13. Your keying tones would then be 0 dbm for high guard tone, function tone at -10, and low level guard at -30. BIMS would be set for that -10 @ 2/3 level, and the keying tones are what they are, but generally follow what I've stated. And that's half of the story. Now you're spitting out the right tone levels, there's just as much to do on the other end of the circuit. I'm running into radios where the line input level is more or less random, and the deviation is set to give 2/3 for -10. so, you plug a tone in at the console, key the radio, and your dev meter says 3.3 (or whatever). But it's not supposed to be in limiting at that level. Set it right, and test tone is below limiting, and you can throw as much audio at the tx as you can, and it'll never over dev. Obviously, simulcast radios running transparent are set differently...

Anyway, that's the way I've always seen it done. YMMV. Having detailed block and level diagrams of every stage from BIM to transmitter is a useful way of showing techs how and where to set levels, and can be real handy in avoiding finger pointing when something doesn't work.

[chartofmaryland]

Please don't forget, any technician, in the new world scheme of things does not get their information from a reputable source, senior tech, college ( they do not teach anything even close to this anymore), manual, or even schooling from the MANUFACTURER ? ? ?.

These so called qualified individuals call tech support for their schooling and then preach the gospel according to their phone call.

Its hard to compete with that,

CoM

[Z Barebow]

I was a field tech for 20 years and this is probably the #1 item which affected real/perceived system performance. I am very particular when it comes to level setting. I have run into many systems who have technicians who graduated from the Helen Keller school of level setting. They obviously could not hear or see anything!

When I was called in to "repair" a system installed by others, this is probably the #1 cause for problems. (It is amazing how a SOB [Some Other Brand] system can sound when audio levels are set properly).

Some items not yet touched on. In a system with multiple remotes, terminate only one remote/console in 600 ohms. (Generally the remote "furthest" from the base). I one came into a DC system with 16 remotes on it, with the customer wanting to add another. Needless to say, this system has problems! (Design/ levels, etc.) It "appeared" as though the system has end to end loss of 26 db. Conventional wisdom leans toward telco issue, (since rule of thumb anything greater than 20db loss is telco issue). But after continued investigation, it was found 7 remotes still had the line termination jumper installed.

A second issue is many of the newer dsp based remote adapters utilize automatic leveling. Techs without proper knowledge will adjust console/remote output thinking this will affect modulated voice. A DSP based tone remote circuit senses the HLGT. With the EIA standard in place, the TRA knows the average voice should be -10 db down of HLGT and +20 db above LLGT. (Frankly, the TRA makes up for lack of level setting knowledge). What ends up happening is the modulated voice will appear normal, but parallel remotes will hear varied voice levels.

Back to TRC systems. I did not have a storage scope, but utilized an audio test set. I would set the BIM output to -10 db with console alert tone #1. I prefered this vs the reset button as the alert tone utilized the tone section utilized for paging tone generation. (more closely emulating real world operation). As referenced above, paging tones are sent out pre-emphasized, (And done so very accurately I might add!). A low freq paging tone can be +6 to +9 db greater than your 1 KHZ reference tone. This will cause compression in MUX's or active leased lines if BIM output were set to 0 db. (Assuming the station will key due to HLGT distortion).

At the station, line input control should be adjusted to achieve +/- 3.75 KHZ deviation w/PL as measured with alert tone generation. (Another reason to use console alert one versus BIM reset tone.). Some nay sayers may say I am not taking into account the composite LLGT with test tone. But when measured, LLGT addition to the test tone is negligible. I have been critqued by a couple of different consultants during acceptance testing and they both commented on how well the systems sounded.

Don't get caught with the +10 db references by some mfgs. This is a relative level in relation ship to FT or average vx level.

When db is notated as a reference, + 10, 0, -20 db is the same as 0, -10, -30 db.

All of these measurements and setting are to be adjusted and set before any microphone level is ever adjusted.

[Jim202]

Very well said. Wish more field tech's would follow in your footsteps. There is nothing worse than
to walk into a strange new to you site and you find that no two radio audio channel levels are
even set the same. This is a true sign of a fly by night, jack legged tech that generally lacks even
the simplest of test equipment and the skill to use them if they are available.

With the way the labor market is today, your lucky to find many field techs that have even put
their hands on any audio level measurement equipment or even know how to use them. It's
rare to run into a radio tech, when I am traveling around the country, that knows the correct
steps to align up a tone remote controlled radio.

Must be the new PLUG and PLAY mode that many radio shops seem to adopt.

Jim

I was a field tech for 20 years and this is probably the #1 item which affected
real/perceived system performance. I am very particular when it comes to level setting. I have run
into many systems who have technicians who graduated from the Helen Keller school of level setting.
They obviously could not hear or see anything!

When I was called in to "repair" a system installed by others, this is probably the #1 cause for
problems. (It is amazing how a SOB [Some Other Brand] system can sound when audio levels are
set properly).