FM Stereo test using 565x

[ltok]

Hi,

I know that I am able to generate FM Stereo signal using PXI 5670. However, I would like to know if I am able to generate FM Stereo signal using PXI 565x (5651 or 5652) modules. If possible, could you also attached a sample vi??

Thank you & Regards

Lee

[Yardov]

Hi Lee,

You can definitely generate FM sine signals with a 565x, however, FM stereo works different and I will explain little more so that you can define if that's what you want.
FM Stereo actually carries two signals, you may think left and right but is actually the sum and the difference. The reason they made it this way was to make it compatible with mono stereos.
Once you have demodulated your FM signal, it should look at the one on the picture:

They send the sum of Left and Right Channels (L+R) at the audible frequencies (about 20 Hz to 18k Hz) so that if we pass directly this signal to a speaker will sound. To produce stereo signal we need the second channel. L-R is modulated AM at 38kHz so they need to demodulate this down to baseband to play it on the second channel. For hardware simplicity, they send a tone at 19kHz that make the FM stereo know if the signal is stereo and help him demodulate it.

Now, to answer your question. You can "fake" that you have a stereo signal if you generate a tone at 19kHz and modulate it FM at a carrier frequency that you want (which you can do with the 565x). If you need to have different signals in the left and right channels, then you will need to generate multi tones which you cannot do with the 565x and you can do with the 567x.

Hope this helps,

Message Edited by Yardov on 05-01-2007 07:02 PM

[ltok]

Hi Yardov,

Thank you for your reply and information. I am very new to this so I hope that you will help me further.

I found an example in NI Website (http://zone.ni.com/devzone/cda/epd/p/id/1575) which is for generating FM Stereo signal through 5670. This example make sure of Configure IQ Enabled and Write Arb Waveform. 565x does not support this 2 function. I would like to know how to generate both left and right signal using 565x.

When you say 'fake' it by generating a 19kHz signal, does that mean that the carrier frequency only have this 19kHz signal and anything else (no left and right signal)?? You also mention that if I want to generate different signals in left and right channels, does having different frequency means generating different signals??

I would also like to know what does Modulation Factor (Main) and Modulation Factor (Pilot) means. Could you refer to the example found in the link below??

I also have a hardware setup, 1 5670, 1 RFSA, 1 DSA card and 1 set of 4 car radio resistors/amplifier (which will be connected to a speaker each). I am measuring voltage across each resistor. The thing that I am puzzled is that when I use the example to generate a signal with mode 'L only', I get the same signal from all the 4 resistors. I thought I should only get signal from the resistors for left speakers in the car. Please help me with this.

Thank you & Regards

Lee

[Yardov]

Hello Lee,

... This example make sure of Configure IQ Enabled and Write Arb Waveform. 565x does not support this 2 function. I would like to know how to generate both left and right signal using 565x.

You cannot use 565x with this example. 565x is a Continuous Waveform (CW) source and not an Arbitrary Waveform (ARB) as the 5421 (part of the 5670).

When you say 'fake' it by generating a 19kHz signal, does that mean that the carrier frequency only have this 19kHz signal and anything else (no left and right signal)??

Yes

You also mention that if I want to generate different signals in left and right channels, does having different frequency means generating different signals??

You would need to generate three signals for FM stereo: Audio + 19kHz tone + Audio at 38kHz and then FM modulate all these signals. At the end, you will have only one signal at the carrier frequency.

I would also like to know what does Modulation Factor (Main) and Modulation Factor (Pilot) means. Could you refer to the example found in the link below??

There are settings of how FM exactly divide the power. You can find good links if you search the internet for "basics fm stereo" or "audio fm stereo tutorial".

I also have a hardware setup, 1 5670, 1 RFSA, 1 DSA card and 1 set of 4 car radio resistors/amplifier (which will be connected to a speaker each). I am measuring voltage across each resistor. The thing that I am puzzled is that when I use the example to generate a signal with mode 'L only', I get the same signal from all the 4 resistors. I thought I should only get signal from the resistors for left speakers in the car. Please help me with this.

This question require that you mention your setup in more detail. Remember that once you demodulate entirely the FM stereo signal, you will have two audio signals (L and R), how exactly are you outputting these signals?

If you have more questions about National Instruments hardware, I will be more than happy to answer them in this post.

[ltok]

Hi Gerardo,

Once again thank you for your reply.

Regarding my setup, I have 5670, DSA (4461), 1 car radio, 1 set of car radio resistors/amplifiers (there is 4 resistors on the set) and an external power supply. This external power supply is to power up the car radio. The car radio has a dummy antenna attached to it. The car radio resistors/amplifiers are also connected to the car radio. So I use the FM Stereo example to generate a FM stereo signal through 5670. I set the mode to 'L only'. The output of 5670 is connected to the open end of the dummy antenna. I take the voltage across the resistors/amplifiers by using 4461. I notice that all the signals I get from across all the 4 resistors/amplifiers (by using 4461) are the same. May I know if this is the expected result?? If so, why?? I do not understand because I am generating in 'L only' mode and the results I am expecting is only resistors/amplifiers for left speakers will have signal.

By the way, the signals I get across resistors/amplifiers are sine wave with the corresponding frequency and amplitude about 0.7.

Is the description above detail?? I am not sure if I am missing anything out. Please tell me if you need another information.

Thank you & Regards
Lee

[parthabe]

Good knowledge base !

[Yardov]

Hi Lee,

Well, I am not entirely sure how the car radio is demodulating the signal. I will try it when I get in touch of a Radio to perform the test. I imagine that the left channel is being also modulated on the L-R area and therefore I think is completely expected what you see. Try using the example "MT RFSG Generate FM.vi" to generate a tone at 19kHz and see if there are any lights in the car radio that light ups indicating that there is a stereo signal present. Then, the voltage on the speakers should be 0 (no signal).
Let me know how this works out,

[ltok]

Hi Gerardo,

I have tested it on my side. I have attached printscreens of the test that I did. I changed the message frequency and kept the FM Deviation at 22.5kHz. Not sure if this value is correct but this is the value for my other FM test (just a normal FM test). For the one at 1kHz, the amplitude changes as I change the volume of the speaker so that is correct (I assume).

Hope you can test it out at your side soon and tell me what you observe. It could be that I have define some wrong value. 😃

Thank you & Regards
Lee

[Yardov]

Hi Lee,

Seems like your car radio (which makes sense) place in both L and R speakers if he only detects mono signal. It would be very annoying to listen only one side if mono was detected (electrically is also easier).
I had a radio and works fine every single mode of the example. I did had to increment the "Modulation Factor (Pilot)" to 90% such as my radio detected FM stereo signal (remember this pilot tone will "tell" the radio that there is stereo information) and therefore uses the L+R and L-R knowledge to figure out what to place in L and R.
As recommendation use, 5k tones in L and R. Then use R-only and make sure you can measure only on R. Perform the same with L and then use "default" which will do both.
Have fun,

[ltok]

Hi Gerardo,

Thank you for your reply. Could you give me some time to try it out?? I will get back to you on what I am getting. Currently I am busy with other stuff.

I really appreciate that you actually spend time to try out my problem. Thank you so much.

Regards
Lee