8Kbit/s sec sampling will be fine. An 8 bit (256 level) ADC will also be
fine for telephone quality - it's what the phone companies use after all!
You don't need 12 bits and 10 bits is probably more resolution than
justified.

For degraded speech quality I think you can drop to ~6 bit resolution and
~6k sampling but it sounds pretty rough!

I'll leave the exact  ADC connections to someone else but you need to use a
5V regulator to drop the 9V supply to 5V for the ADC;  a resistor will not
be sufficient.

The phone line will also need to  be isolated from the "outside world". If
the kit is in a plastic box along with the battery, you may be able to get
away with it (at least for private use !) but for any external connection
(eg the ethernet link) or sellable product  you will need to provide an
isolation barrier (and some sort of compiance/approval) depending on your
market.

Don't forget the 100Vac or so you get back down the line when the phone
rings - you will need to provide protection for this also!

Richard P




Hello,

I have a few questions in regards to ADC capable PICs (in regards to
the project mentioned below). Unfortunately, my knowledge is lacking
although I am trying to correct that), so bare with me.

I am wanting to sample input from a telephone, encode that input into
mu-law audio format, and then send that data over a LAN to another
machine. I found a company that sells boards that use the CS8900
ethernet chips (preassembled and kits), as well as a tutorial on how
to send UDP packets over an ethernet link using it. Right now however,
I am more concerned with sampling the signal from the telephone, and
to make sure my basic design for that is correct.

Since human speech usually is below 4000 hz, the minimum sample rate I
would need is 8,000 samples per second. Since the signal itself's
highest frequency should be around 4000 hz, a 12 bit resolution (4096
possible values) should work. Is this correct?

I have been looking at the PIC16C77X MCUs.. I was also looking at the
PDF datasheet at:

http://www.microchip.com/download/lit/pline/picmicro/families/16c77x/30275a.pdf


So here are some more somewhat basic questions:

The pinout diagram shows pins 11 as AVdd, pin 12 as AVss, pin 4 as
VRef- and pin 5 as VRef+. These are all of the pins I will
need to use specifically for the ADC part of things, correct? I wish to
have a telephone connected via a standard telephone cord to an RJ11 jack
on a standalone unit (with among other things the PIC in it). A 9V power
supply will be used to power the phone line. Since 9v is way too much to
connect to the AD on the PIC (correct?), a resistor will need to be used
to get it below the 4.096V level (which is the max voltage level for
AVDD,AVSS right?).

So, basically I was thinking the negative terminal on the battery can
be connected to the RING pin on the RJ11 jack, and the TIP pin on the
jack can be connected to the Avdd pin on the PIC, and have the AVss
pin connected to the positive terminal on the battery... I would need
a resistor inbetween the RJ11 TIP pin and the AVDD pin (correct?)..

I have the bad feeling that I have something (lots of things?)
backward however (hence the reason for this post).. I also don't know
what to use for VREF + or VREF -... In the datasheet it mentions "The
analog reference voltages are software selectable to either the
device's analog positive and negative supply voltages (AVDD/AVSS),
the voltage level on the VREF+ and VREF- pins, or the internal
voltage reference if available (VRH, VRL)." Does this mean that only
the AVDD, and AVSS pins are necessary, and VREF+ and - are not needed,
assuming I set the reference voltage to the analog positive and negative
supply?

If someone would care to point out the mistakes I made above, point me
in the right direction etc.. that would be great!

Nathan

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu