Audioguru
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As per notes in the link that agree with me, you made a sidetone elimination circuit which is used to reduce feedback in a speakerphone, not an echo canceller.I made this with exact values and did not work for me.
Here, sidetone elimination circuit is required not echo canceller. I came up with this, see in simulation. At a certain setting The audio on line is separated.
Upper trace is speaker input from GSM. Second trace is from Skype. Third trace is mixed audio on line. Fourth trace is extracted audio for GSM mic input.
Adjust level of input/output to appropriate level being not too loud that it is distorted. Then adjust for deep null.
Any PNP transistor with power handling near 1W or more will do, such as BD136.
Capacitors are +ve towards LM358 and negative towards GSM connections for both input and output. There will be no current in transistor while line is not connected. Line will draw about 45mA. Check it with current meter. There will be around 7V on line. Check it with voltmeter. Don't forget to put filter capacitors on in ad out of regulator IC 7818.
Yes, there should be no audio without power. This may be caused due to the fact that GSM speaker output is sometimes a differential output. The ground connection should be connected to chassis (Common negative ground plan) and only one wire to speaker terminal. Similarly only connect one wire to mic, ground connection will be the same. The audio should not leak through ground loops. Your power supply circuit is OK.without power (seems connection point makes some reflection or something
Yes, there should be no audio without power. This may be caused due to the fact that GSM speaker output is sometimes a differential output. The ground connection should be connected to chassis (Common negative ground plan) and only one wire to speaker terminal. Similarly only connect one wire to mic, ground connection will be the same. The audio should not leak through ground loops. Your power supply circuit is OK.
R7 and the other resistance on emitter of transistor are of equal value. They are in series. Both should heat up equally. You did not mentioned the current drawn by line. At 50mA, voltage drop on resistor be around 3V. Ohm's law tells the power dissipated in resistance= VxV/R= 9/100 = 0.1W.
Check voltage drop on these resistances. It seems the other 100 ohm resistance is shorted so that emitter is receiving full +ve voltage.
Make connections to GSM like this.
R1 can be eliminated.100k Variable with series capacitor in place of10k. It will not change basic performance of the circuit only due to availability of components. Her you can see expected voltages on different nodes. R7 here is not actual but represent load. LED will lit when line is off-hook.
Variable resistor will not change voltage on any pin but eliminate echo at certain setting.
R1 can be adjusted to set line voltage and current of line. You can hook ordinary phone with line output instead and observe audio quality and echo cancellation. Actually I am going to use this device as interface between PC and landline. If some relative is on SKYPE then I can transfer or make a conference call with another person who is not on SKYPE by dialling his number from landline phone and transfer line to SKYPE using this interface connected to headphone/mic of laptop.
Telephone sets have such type of circuit built-in. You can experiment connecting phone to line output of freetalk and connecting speaker output and mic input from its cradle to GSM without using any extra circuit.
Try removing 33k in series with 100k to get further adjustment. 100k variable resistor may require carefull adjustment.
Do experiment with ordinary phone in place of freetalk and observe the audio on caller's side.
When echo is less, further adjust gain of input/output variables again. Repeat the process two three times to find optimum setting.
These circuits demonstrate the basic principle that to feed equal amplitude but out of phase signal so that they cancel each other and the wanted signal remains.
Put a 100uF capacitor in parallel with 100 ohm resistor on emitter of transistor. Play with setting again.
First disconnect mic and adjust so that SKYPE listener hears GSM caller clearly. As mic is not connected there will be no echo and feedback. Now connect mic with low volume setting. GSM listener will hear SKYPE caller. Both will hear some echo. Reduce echo with variable. Increase mic volume and again adjust echo again.
Can you post some pictures of your circuit assembly and connections setup?
Then you will be making a very simple and cheap speakerphone that has "voice switching". Motorola made two voice switching speakerphone ICs in the early '80ies.Is it possible to add some transistors switches to receiver and transmitter with fine tune?
Many phone calls have people talking at the same time but voice switching allows the other person (or noise) to hog or break up the voice direction.Usually in a telephone call both sides are not talking same time.
People do not like voice switching because it cuts off parts of words. That is why modern full duplex speakerphones with echo canceller circuits are made and sold.When GSM mic input received audio signal then transmitter (Skype caller audio) should disconnect and the opposite. If those are switches well then it should like real conversation.
How will any audio pass through, from GSM, to GSM caller, when there is no audio on its mic input.When mic (GSM) is not connected skype caller audio never comes clear.
If there is no echo on GSM side then GSM speaker side does not hear anything. I tried remove mic and connect speaker, then GSM side can hear quality audio without echo. But when I plug mic terminal echo comes.
I find this confusing to understand.
How will any audio pass through, from GSM, to GSM caller, when there is no audio on its mic input.
For example, you make a Skype call, put him on hold . Now you want to connect this call to another person on a distant location with a phone with him. You dial through your GSM to that phone and when call is established, you connect the Skype caller, who is on hold, through this device and the conversation begins.
Lets say there are three persons. One is on mobile phone end, second is you with GSM and freecall device and third is on Skype other side.
It should be like this, if mic is disconnected, the person on the phone ( GSM caller) will not hear sound from Skype freecall.
The person on Skype will hear the person on phone because there his speech is passing through speaker of GSM to the Skype. There is no feedback so the person on Skype(Skype caller) will hear the person on phone clearly.
Put a 470pF capacitor in parallel to 18K resistor between pin1 and 2 of the IC. I will assemble mine tomorrow and test it.
I found another full duplex audio circuit. Please have a look too.
**broken link removed**
One important point is worth mentioning. FreeTalk device has two outputs. On one point an ordinary phone is connected. On second point, telephone line is connected. This device we made is to be connected with the point where landline is connected, not with telephone output connecter.
Oh! that's why it is not working. Let me redraw it for work as regular phone.
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