[App_rpt-users] usbfobs

Scott Zimmerman n3xcc at repeater-builder.com
Wed Jul 6 09:09:57 EDT 2016


 > If you look at this close, it is just a glorified op-amp.
 > We used a 741 for dsp on a serial line in the 90's.
Huh? If you think the CM119 is simply a glorified op-amp, you've got BIG issues!

I'd also like to hear more about how you used a 741 on a serial line to do DSP functions. Maybe all 
the rest of us have been doing it wrong for the past few decades. ;)

What you are missing is that the I/O lines are not the big stumbling block to designing an 
alternative to the CM119. Yes, I do agree if there were alternative sources of PTT, it would allow 
use of the 'potted' variety of sound FOB, but the root cause still remains.

 > Another excellent question.  This I2S needs looking into.
I have posed this question to folks that are smarter than I, and they all tell me that the CM119A 
best fits the bill based on several factors:

1) The "A" version of the CM119 requires a crystal this makes it frequency stable. (for the most 
part) The "B" version has an internal R/C oscillator that WILL drift over temperature and other 
environmental factors. This drift can be large enough to skew the sample rate enough that the LTR 
and other protocols that require accurate timing may no longer work. This could include CTCSS 
signalling.

I think USB bus timing plays an important role in the "big picture" as well. I'm a bit fuzzy on 
this, but I think I remember someone telling me that the USB buss timing plays an important role in 
keeping the CM119's sample rate constant. One would assume I2C could handle things, but I'm thinking 
there was an issue with I2C and sample rate(s).

2) Most ADC designs include a high-pass filter ahead of them. These filters are designed to remove 
"rumble" and any DC bias that exists on the input pins. In the world of communications, this 
"rumble" is known as low-speed data signalling! The CM119 was designed to be cheap and therefore no 
high-pass filter.

3) PRICE - what does an I2C device that will do DC type A to D conversions cost? The ones I found 
that might work were in the $5 range from US distributors. With the more expensive part, you STILL 
have to add on the signalling and re-write drivers to work with it.

I'm not trying to quench people's ambitions to start down a new path, I'm simply passing along my 
findings as I researched potential alternatives.

Scott

Scott Zimmerman
Amateur Radio Call N3XCC
474 Barnett Road
Boswell, PA 15531
On 7/5/2016 10:38 PM, mike at midnighteng.com wrote:
>
> If you look at this close, it is just a glorified op-amp.
> We used a 741 for dsp on a serial line in the 90's.
>
> What makes it tough is the I/O lines some use for PTT/COS.
> and the address lines for them that may not be the same with other units.
> Could just change the source code for some others.
>
> I suggest finding a way to replace the I/O lines with something else
> because ANY dsp should work fine for the audio.
> I think the PI itself has them. Just have to write the code to replace it
> OR use "ON-EVENT" programming to read/write to them to follow ptt/cos.
>
> I know that is easy for me to say as I'm using Atom boards with a parallel port.
> And there are plenty of I/O / DAQ boards out there that can be utilized for plenty
> of I/O if the PI's I/O can't work for it.
>
> Just remember, it was the work of others that made it so easy so far.
> But a good thing never lasts forever.
>
> ...mike/kb8jnm
>
>
>> drivers?  hardware?  integration with existing codebase?  takes time and (unpaid)
>> effort.
>>
>> --
>> Bryan
>> Sent from my iPhone 6S...No electrons were harmed in the sending of this message.
>>
>>
>>
>>> On Jul 5, 2016, at 20:54, Steven Donegan <donegan at donegan.org> wrote:
>>>
>>> I would ask a different question - why do we still use slow USB devices rather
>>> than ones directly attached to the Pi's bus?
>>>
>>> Steven Donegan
>>> KK6IVC General Class FCC License
>>> FAA PP-ASEL Pilots License
>>> Silver State Car #86
>>> www.sscc.us
>>>
>>>
>>> From: David McGough <kb4fxc at inttek.net>
>>> To: app_rpt mailing list <app_rpt-users at ohnosec.org>
>>> Sent: Tuesday, July 5, 2016 5:40 PM
>>> Subject: Re: [App_rpt-users] usbfobs
>>>
>>>
>>> Steve,
>>>
>>> The CM108/CM119 is nowhere near dead! Even if CM stops manufacture, there
>>> will be decent "clones" of it for years to come. No worries about that!
>>>
>>> If you're needing some FOBs, I understand the shipping issues you've got,
>>> being in NZ. I just checked the US Postal Service shipping site and a
>>> "First Class Mail International Large Envelope" is under $20USD for 2
>>> lbs., shipped from the USA to NZ.  Priority mail, where you could ship a
>>> small box in under a week, starts at about $64USD.  See:
>>>
>>> https://ircalc.usps.com/?country=10324
>>>
>>>
>>> Why not just work a "bulk quantity" deal with Kevin or Scott to sell you
>>> and ship you a hand full of their RIM-lite adapters???  Then, you'll in
>>> good shape for now.
>>>
>>> http://www.repeater-builder.com/products/usb-rim-lite.html
>>>
>>>
>>> 73, David KB4FXC
>>>
>>>
>>> On Wed, 6 Jul 2016, Steve Wright wrote:
>>>
>>>> On 06/07/16 11:44, Kevin Custer wrote:
>>>>> Soldering a 48 pin LQFP and having it be fake is nothing short of a
>>>>> bad experience.  Like Scott, I recently purchased a fair quantity of
>>>>> CM119A's from a top CM distributor.  They told us the CM119 and CM119A
>>>>> are now legacy products and the CM119B is current. Unfortunately, the
>>>>> B model doesn't use a crystal and several developers agree that they
>>>>> may not be suitable for digital and analog radio adapter
>>>>> applications.  There were still a few hundred CM119A's in the
>>>>> distribution chain as of a few months ago.
>>>>
>>>> If the CM119 is dead, then we should scrap it.  I can't really see why
>>>> we are focussing on that devices' built-in I/O, when we have computing
>>>> devices such as RPI3 with scads of easily accessible I/O.
>>>>
>>>> Linux sound is really well developed - "just works everywhere" I'd say.
>>>> Why not build or reuse a DSP-based driver for that, that will handle
>>>> voting - now we are in business with all the toys, for the foreseeable
>>>> future.  Likely a simple CLI config tool could be developed too - choose
>>>> RX audio device, TX audio device, signalling, save settings, done.
>>>> Rinse and repeat for three devices.
>>>>
>>>> Hardware VOTERs are fine for a single repeater with a few outboard
>>>> receivers, but way too much money for non-millionaire clubs to convert
>>>> "everything" to asterisk - they will stick with their IRLP node for
>>>> sure, and that is a tragedy..  I see a few members here and elsewhere
>>>> make it their business to put tens of thousands of $$ into communication
>>>> systems, and that's enviable and laudable, but 90% of constructors don't
>>>> exist on that rarefied plane.
>>>>
>>>>
>>>> S
>>>>
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