Audio for interphone with RS485

Thread Starter

giovanni.dista

Joined Apr 14, 2021
45
Happy Saturday everyone, I would like to design an intercom using the rs485, the maximum distance that the signal must travel is 200 meters and the audio quality must not be high. I saw that the spdif has a high bitrate and for my project it is not needed, how can I do it? ( not use Dante)
 
Last edited:

Thread Starter

giovanni.dista

Joined Apr 14, 2021
45
Hi Ian thanks for the reply !! if I use a cat 6 cable and a MAX22500E transciever, seeing figure 9 of the document you kindly posted to me, I can transmit at 20 Mbit / s at 200 meters right? (the figure expresses the speed in meters)

However, I have a doubt, as I wrote you in the previous post, using an audio encoding spdif 44.1 Khz I should have a double bitrate than the one you wrote because spdif uses the Biphase Mark Code right?
 

Ian0

Joined Aug 7, 2020
3,268
Biphase mark does double the bit rate, but it also synchronises the clock every bit, so it's not quite as bad.
Anything involving PCM (such as S/PDIF) will get messed up if data bits get lost, because they are part of a binary number. Something like delta/sigma will fare much better.
If it's an intercom, what's wrong with analogue? After all, the local loop of an ordinary telephone can manage intelligible speech over at least 10km. It's only when you try to superimpose digital signals on top of it (broadband) it gets into trouble.
All you need is an amplifier capable of driving 220 ohms - for a cat-5 cable terminated at both ends. In fact, you could use cheaper telephone cable. Similar charactistic impedance.
 

Thread Starter

giovanni.dista

Joined Apr 14, 2021
45
unfortunately the project specifications are to use an intercom using the RS485 and digital audio, but I can do it with the max2250e and a cat6 cable and also have a good audio quality even if it is not the priority at least so I understand from the paper.
 

Ian0

Joined Aug 7, 2020
3,268
So this has degenerated into a pointless exercise of redesigning the telephone circuit which has worked perfectly-well-thank-you-very-much for over a century? If so, don't waste too much of your valuable time on it. Use @crutschow 's delta-sigma modulator circuit from https://forum.allaboutcircuits.com/threads/serial-audio-transmission-over-cable.111867/post-865452. Put your RS485 transmitter and receiver between U1 and R4. It will be ten times easier to get working than the all digital solution.

For you next task, they'll probably want you to reinvent the wheel - except this time can't you make it a bit squarer? Then we can get more into the shipping container from China.

This is what happens when empty-headed bureaucrats write the specs instead of engineers.
 

AnalogKid

Joined Aug 1, 2013
9,361
Wally's circuit in that link is PWM. As I said in that thread, I suggest CVSD. Motorola has excellent app notes on using their dedicated CVSD chips that are an excellent tutorial. AND - CVSD wan developed by the phone company for exactly this application.

ak
 
Last edited:

Thread Starter

giovanni.dista

Joined Apr 14, 2021
45
So this has degenerated into a pointless exercise of redesigning the telephone circuit which has worked perfectly-well-thank-you-very-much for over a century? If so, don't waste too much of your valuable time on it. Use @crutschow 's delta-sigma modulator circuit from https://forum.allaboutcircuits.com/threads/serial-audio-transmission-over-cable.111867/post-865452. Put your RS485 transmitter and receiver between U1 and R4. It will be ten times easier to get working than the all digital solution.

Hi ian, the solution you proposed to me with PWM modulation is very interesting. However, is there a more performing modulation? what problems could that circuit have? I would like to propose it to my boss!!
Tks a lot Ian for your help!!
 

Ian0

Joined Aug 7, 2020
3,268
Delta-sigma modulation sounds fancy. it’s not much different to @crutschow ’s PWM circuit (in fact I though his was delta-sigma at first glance). I’ll draw it out later. . .
 

Thread Starter

giovanni.dista

Joined Apr 14, 2021
45
Thanks Ak, but because there are so many solutions on the net I ask if there is one more suitable for audio .
sorry if I expressed myself wrong. for example I thought about MAX4734 because I have to multiplex between 4 audio signals.
 
Last edited:

Ian0

Joined Aug 7, 2020
3,268
Delta-sigma modulation sounds fancy. it’s not much different to @crutschow ’s PWM circuit (in fact I though his was delta-sigma at first glance). I’ll draw it out later. . .
https://wiki.analog.com/university/courses/electronics/electronics-lab-17
This is the most basic form. There are fancier version with stuff like noise shaping, and there are second-order modulators which do a better job
A simple improvement would be a comparator between op-amp and 74HC74.
If the RS485 receiver is TTL (i.e. a 75176 or similar) then a CMOS buffer before the filter would be an idea, but if the receiver has a CMOS output stage, it might be unnecessary.
The highest frequency down the cable is half the clock frequency.
The resolution starts at 1 bit at half the clock frequency, and increases at one bit per octave with decreasing frequency.
B56BE1F5-560B-4CA6-A159-4D914EF4343B.jpeg
 
Last edited:

Ian0

Joined Aug 7, 2020
3,268
thanks everyone for the help! I have a question: if I have multiple audio sources how can I multiplex them?
Do you want to put multiple signals down the same cable simultaneously? (That would require more bandwidth) or do you just want to switch between signals?
The professional version of S/PDIF (called AES3) can get 4 signals down the same cable at the same time.
Of course (and you won't want me to mention it) DANTE can get an awful lot of signals simultaneously down ethernet!
 

Yaakov

Joined Jan 27, 2019
3,517
my case and many points that communicate with one instead from what I understand the multidrop is the exact opposite
If that was a response to my deleted message, I removed it because I reviewed the thread and saw that I had missed some things along the way. Sorry.
 
Top