Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

How to detect the "busy nois"

Not open for further replies.


Member level 5
Jan 25, 2005
Reaction score
Trophy points
Activity points
Can you help me to detect the "busy nois" in phone line with pic"

its abiy complex but if you could some ditinguish between the niose and othe voices electricaly it would be good

the busy tone is periodic and has a determined time for make and preak ( usually 0.5 sec. and 0.5 sec.) (may vary from country to another)

so, u have to detect the voices in the telephone line and convert it to logic signal (0 and 1 ) ( if u use a microcontroller) then u can sample this signal, then if this signal has the time properties like standard busy tone so u detect the busy tone.
you have to chek it 3 or more times to avoid errors

hope that help u
Ahmed :)

It looks like Q.35 does define the busy tone parameters , but dont believe to that - operators sometimes can change tone defaults from standard to something their managers love tro hear).
I witnessed that few times

For all measurements you should define smallest time interval to detect noise to decode tone pulse pattern . Let say, we can approach 100-200 milliseconds .

Actual implementation could vary dependent on what quality extend you are willing to achieve

a. Simplest :
You can implement simple comparator connected to to your telepone pair (AC bypass ) and count number of zerocrosses - that will give you frequency of signal in line .

b. Improved :
use 1 bit Goertzel algorithm to detect frequency . A lot of implementations do exist . There is some explanation :
**broken link removed**
**broken link removed**

c. Better than above :
Implement digital filter in PIC to detect narrow frequency (band pass filter) and use filtered data for detection . Let say atmel has an application note
where second order band pass filter on mcu is described. You should find one which is applicable to PIC.

d. Probably other methods such as correlation based could also be used , it is up how you are going to spent your time).
let say this link and many others .

Using above methods you will have a sequence of bits or tone pattern - duration of tone and pause . This is to be matched towards your operator's used tone pattern , which has to be apriori analysed and stored into your program .

This document could be useful but it is little bit complicated:
**broken link removed**

At the end , i suppose seaching google will help to find ready to use solution already available,
but i leave this opportunity to you)

This guy has made a dtmf decoder you can use as base to detect busy tone :

good luck

Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to