how to input I/Q signal to soundcard for DSP

[aliumair926]

i/q signal

i am currently studying for my project ,ie to make a fm receiver using PCs soundcard ... as we can demodulate signal with relative ease converting it to I/Q ...the problem is how can i input 2 signals which are "I" and "Q" to one mic input....or is there other way ?? my notebooks soundcard has 3 input ,..one mic one speaker and 3rd one i kant figure out wotz that for..manual says that it is line in to accept audio from other devices...is it the same tht i can use for I/Q input?...plz help...thnx

 

[mowgli]

dsp i/q

Instead of sending I and Q (I don't think it is possible because you can chose only one input channel between MIC or Line In) try to down convert the FM signal down to an Intermediate Frequency (IF) centered around 8-10 kHz and send this signal to the line in input of the Sound card to do the A/D conversion, then I and Q could be generated digitally with a Software Numerically Controlled Oscillator (NCO).
In theory it seems easy and so it is if you have a signal generator which produces directly the IF FM signal with suitable frequency, amplitude and bandwidth, but if you want to demodulate a real signal this is not so easy. First it shall be a narrow band one: I don't know exactly the cut off frequency of the sound cards line in input but I suppose it should be around 17-20 kHz so you have few kHz bandwidth available for a IF at 8 - 10 kHz (few if compared to modern digital communication systems, with bandwidths in the order of the MHz).
Then you have to design a RF to IF conversion stage with output levels capable of driving the sound card input (or try to assemble it using laboratory components as adjustable bench filters, mixer and generators).
And finally for what concern the signal processing on the PC using Matlab much work is for free (except the cost for the license ;-) ) because you should be able to acquire IF samples from Sound card and process them in Matlab, but I am not sure it is possible to do this in "real time" (= acquisition while processing like commercial software oscilloscopes do) or only off line, alternating acquisition (e.g. recording of a finite length duration wav file) of a tranche of the IF signal and processing (import in Matlab the wav and apply demodulation algorithm to the encoded samples).

Regards
Mowgli

 

[bepobalote]

sound card i/q receiver

If your MIC input is mono you can't sample I/Q signals.
Please check if your Line-in input is stereo: if yes it will be OK for your requirements.
By the way, if possible, it is better to use a line-in input instead of a mic one: the last one is in general noisier and sometimes not linear or even compressed to accomodate non standard inputs.

If possible do not write like you were sending an SMS: not everyone will understand the abbreviations you used!
Your message is not limited to 160 characters and fortunately you are using a real keyboard, so please write in plain english! :wink:

 

[aliumair926]

sound card input signals

@ bepobalote: You mentioned "if your Line-in input is stereo: if yes it will be OK for your requirements. " Does this mean that there should be two input jacks (line in)
as i have to input 2 signals (I and Q)....

@ mowgli: Sir will i be able to do real time processing in simulink!...

 

[bepobalote]

pci 8-input sound card 196 khz

@ bepobalote: You mentioned "if your Line-in input is stereo: if yes it will be OK for your requirements. " Does this mean that there should be two input jacks (line in)as i have to input 2 signals (I and Q)....

No, you will use the Left/Right channel as I/Q, so you will need a stereo jack.

here there are some links which can be of interest to you: they are related to some SDR receivers...

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http://www.dspguru.com/info/tutor/quadsig.htm
http://www.arrl.org/tis/info/sdr.html
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if you need more references, do not exitate to ask.

P.S. I think you will be able to demodulate only Narrow FM signals: with a standard sound card it will be impossible to demodulate Wide FM (like radios from 88 to 108 MHz)

P.P.S. you can try the wonderful program (winrad) at the original web site: http://www.weaksignals.com/ or a newer version at http://www.winrad.org/: to my knowledge it decodes even FM transmissions. By the way, you can get even source code for the program: maybe it will help you!!! :wink:

 

[aliumair926]

sdr radio for the masses schmatics

Thanks alot for your concern.. But my project is to make a PC based FM receiver (to receive FM radio channels and that would be from 88 to 108 Mhz) capable of receiving FM broadcast in realtime....

What do you suggest me accordingly?...is simulink suitable?

 

[bepobalote]

150khz sound card

The problem is the Wide FM bandwidth: if I correctly remember it is 150KHz + 2x25KHz of guard bands.
The BEST soundcards sample at 192KHz which gives you wery little "space" to work with...
Your notebook is surely limited to a MAX 48KHz sampling rate!

I have never used Silmulink, so I can't help you on this side.

 

[FvM]

sound card bandwidth

But my project is to make a PC based FM receiver (to receive FM radio channels and that would be from 88 to 108 Mhz) capable of receiving FM broadcast in realtime....

bepobalote has stated the problem very clearly. If you use a smallband FM receiver for wideband signal, you get a seriously distorted signal. I wonder, if there's a project instructor with technical background, who checks the specification?

 

[mowgli]

pc sound card input bandwidth

As a general consideration in receiver design you have to consider requirements coming from both the digital and analog domain.
As I stated in my previous post you have to check the maximum bandwidth for the analog input: the Sound card Line In is not adequate for 200 kHz channels of FM broadcast radio. In my opinion in your case the limiting factor is not only the maximum sample rate of the sound card, but the limited bandwidth of the input front end: you could even have MHz sampling rate on the sound card but if the Line in cut off frequency is lower than 20 kHz you loose information in the analog domain in any case! And this is the case: the maximum AD frequency of the sound card (196 kHz) is dimensioned to work with audio signals, whose bandwidth is about 20 kHz!

For what concern Simulink I don't know if real time processing could be possible: for sure I know that Matlab is capable of processing wav signals but I have never tried this feature.

Regards
Mowgli

 

[bepobalote]

sound card output i and q

In my opinion in your case the limiting factor is not only the maximum sample rate of the sound card, but the limited bandwidth of the input front end: you could even have MHz sampling rate on the sound card but if the Line in cut off frequency is lower than 20 kHz you loose information in the analog domain in any case! And this is the case: the maximum AD frequency of the sound card (196 kHz) is dimensioned to work with audio signals, whose bandwidth is about 20 kHz!

In the links I gave, you can find references to some sound cards which bandwidth is 1/2 of the sampling frequency (like EMU 1212M at 192 KHz).
One other problem to be considered is the behavior of the ADC: a lots of them are really "noisy" at frequencies higher than 30/40 KHz (alias noise). One of the BEST audio ADCs is the **broken link removed**.
Take also a look at: http://hpsdr.info/wiki/images/4/46/Janus-DCC-2006-paper.pdf

 

[FvM]

perseus,digital demodulation

There are also several TI audio ADCs with fs >=192 kHz available, e.g. PCM 1804, PCM4202.

The said "noisy" behaviour is a result of the SD-ADC noise shaping. But the TI audio ADC still achieve 12-Bit SNR over the full bandwidth. Also the filtering isn't that steep with 192 kHz sampling frequency, so some aliasing around fs/2 must be expected.

I didn't check available audio cards specification. But if they don't further restrict the ADC bandwidth, wideband FM seems basically feasible with these parts, although I don't think they are first choice for a SDR design. Basically, a higher sampling rate would be required for a more flexible IF filtering.

 

[bepobalote]

i/q inputs

FvM, you are right, but I was talking only about available sound cards and I didn't want to be too detailed...
If you need more bandwidth, you can use SDR receivers like **broken link removed** or **broken link removed**

Here you can find some references to different types of SDR receivers:
**broken link removed**

 

[aliumair926]

signal generator with soundcard matlab

...Wide FM bandwidth: if I correctly remember it is 150KHz + 2x25KHz of guard bands. .

first of all thanks alot to you all for your replies...
ok when u talk about "150KHz + 50 KHz of guard bands" i get a bit confused about one thing...
Can you please tell me that are you talking about some particular channel(like FM 100 MHZ) or complete FM band (88-108 MHz)..

Secondly if its channel then i wont be able to tune it in software,.(only)...and if i get a soundcard capable of sampling 96Khz then how much of bandwidth (wide FM) will it cover for reception.

 

[bepobalote]

matlab soundcard digitize input

For 150KHz, I meant that every FM channel in the 88-108MHz band is using 150KHz of band.
The 25*2 "guard band" is some space between any of those channels to prevent interference between near channels.
Take a look at the following page: https://en.wikipedia.org/wiki/FM_broadcasting for a description of the usage of a FM channel.

I really don't know how you will be able to tune in SW your SDR radio: it depends on your receiver hardware.
I imagine your HW is something like a downconverter which shifts the received frequency to a lower one (Hetrodyne like).
Maybe it will be better to check with your tutor...

 

[aliumair926]

iq demodulation soundcard

Yes , you are right , i will be using downconverter to shift it down to some IF. Well yes i a working on it , as the available hardware is also an issue....
i have gone through A Software-Defined Radio for the Masses. By Gerald Youngblood, ....In that he has used Tayloe detector and other circuits ..
I hope i can build that...

**broken link removed** this link has Yblood1, Yblood2, Yblood3...
do you have any idea if any body has tried doing it...[/url]

 

[bepobalote]

sound card adc converter project signal

If I remember correctly, the receiver shown in "A Software-Defined Radio..." will not work correctly because there are some errors in the schematics (DDS filter), but there are lots of working implementations of SDR radios.
I suggest you to take a look to the Softrock family:
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http://www.softrockradio.org/
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http://ewjt.com/kd5tfd/sdr1k-notebook/sr40/sr6-production/index.html
http://www.ewjt.com/kd5tfd/sdr1k-notebook/sr40/index.html
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This family of receivers/transmitters is REALLY cheap.

You can also subscribe to the Yahoo! Softrock40 group and make a search in the archives: you were not the first one who tried to do what you need.

Personally, I will suggest the following:
1) take a cheap FM receiver
2) identify its IF [Intermediate Frequency] (in general it is 10.7 MHz)
3) order a Softrock with the right receiving frequency (same as the IF of FM receiver one)
4) tap Softrock to the IF of the FM receiver
5a) Buy a GOOD soundcard (I suggest EMU 1212M, not cheap but really good) with a fast ADC (at least 192Ksamples/s)
5b) Otherwise try to find a double A/D converter (on PCI/...) with at least 250Ksamples/s. I think a 10/12 bit resolution is enough, but do not trust me on this...
6) start to develop your software.

7) good luck!!!!

 

[jumpjack]

I just found out that my PC audio card is capable of sampling up to 3MHz sampling frequency 8O . And I think I cuoudl even put it over this value .
Although it's useless for sound sampling, it's very useful to use audio card as an oscilloscope... but I'm currently using Audacity, an audio editing program, to examine signals! As it only shows up to microseconds, where could I find a free software which allows analyzing WAV files sampled at 3 MHz or even over?