Why is AGC gain at max when input below noise threshold?

Hi Noland,

Even when the LAGC signal is lower than the noise threshold, the signal (noise in this case) will try to reach the target level. That's why the AGC gain is at maximum.

However, the noise signal must be under the noise threshold to be limited. You may try placing the AGC max gain at 0dB to see the noise value. Then, you will need to place the noise threshold over that value to avoid any noise amplification above the limit (see the following pictures for details).

Noise threshold under the noise signal.

Noise threshold above the noise signal.

I hope this helps you. If you still have questions, please let me know.

Best regards,

Luis Fernando Rodríguez S.

Thanks for the quick answer.

"However, the noise signal must be under the noise threshold to be limited." I can see from your screen shots that your noise signal is never limited. Your AGC gain is always high or maxed out. In the 4th image, your signal is below the threshold but the AGC gain is still at 40dB. Why is it applying high gain to noise? The datasheet says it should go down to 0dB.

From the codec application reference: 

"Noise threshold defines the level below which if the input signal level falls, the AGC considers it as
silence, and thus brings down the gain to 0dB in steps of 0.5dB every fS and sets the noise threshold
flag. The gain stays at 0dB unless the input speech signal average rises above the noise threshold
setting."

The gain should be brought down to 0dB if signal is below the noise threshold, correct? Even with the input grounded and a high threshold, I cannot get the gain to drop to 0dB. Can you send me a screen shot of your AGC gain at 0dB and your max AGC gain at 58dB?

The only time I see your AGC gain at 0dB is when you set your max allowable gain to 0dB.

Thanks for your help!

Noland

"Even when the LAGC signal is lower than the noise threshold, the signal (noise in this case) will try to reach the target level."

Isn't that exactly what the noise threshold feature is supposed to NOT do?

From the app reference: "This noise-gating ensures that noise is not 'gained up' in the absence of speech."

I must not be understanding something here....

Hi Noland,

I apologize for the confusion. The AGC behavior can be understood making some tests with the GUI. As you can see in the last pictures that I posted, in both cases there's a noise signal around the -45dB (this is the noise signal without gain). Then, when we configure the AGC Max Gain at 58dB, the noise signal tries to reach the target level.

The datasheet mentions that when the signal is under the noise threshold, the AGC gain stays at 0dB. Maybe this is not explained properly. The AGC gain is applied even if the signal is under the noise threshold, but the signal will never be above the original signal. For example, in the second picture, the original signal value was around the -45dB. When the max gain is applied, the signal stays under the -45dB, even if the gain is around 58dB.

I hope this helps you. If you still have questions or comments, please let me know.

Best regards,

Luis Fernando Rodríguez S.

From what you are saying and from my experiments with the GUI, the noise threshold feature does not work at all. There is no effect on the signal or the AGC behavior if I uncheck or check the "Enable noise threshold" checkbox. 

If the AGC still applies maximum gain to a signal that is under the noise threshold then this is the same as not having any noise threshold.  Are both the datasheet and the application reference wrong when they say AGC gain should step down to 0dB?

Can you please give me some steps to demonstrate when enabling or disabling the noise threshold makes any difference?

"The AGC gain is applied even if the signal is under the noise threshold, but the signal will never be above the original signal." The output of the PGA amp is very much above the original input. It's amplified by 40dB.

Thanks very much! Sorry to take so much of your time!

Noland

Hi Noland,

There's no reason for apologize.

I made some tests in order to explain the AGC behavior:

Test 1: Default configuration.

Using the default configuration (pressing 'ok' in the window with the question: Program the device for this feature?), the signal was limited by the noise threshold.

Test 2: Disabling the noise threshold.

Using the same configuration, the next step was disable the noise threshold. The signal level increased.

Test 3: Using the noise threshold with the AGC max gain at 0dB.

The noise threshold was enabled again but the AGC max gain was changed to 0dB (this would be the original noise signal). The signal is above the noise threshold, so it is not limited.

Test 4: Disabling the noise threshold with the AGC max gain at 0dB.

The noise threshold was disabled and the AGC max gain was conserved at 0dB. No changes observed because the noise signal was greater than the noise threshold in the last test.

Test 5: Using the noise threshold with the AGC max gain at 58dB.

The signal was limited to the noise threshold even if the AGC gain was configured at the maximum. 

Test 6: Disabling the noise threshold with the AGC max gain at 58dB.

Due to the high gain, the signal reached the target level without any limitation of the noise threshold.

I all the cases, the signal will try to reach the target level. But when the noise threshold is enabled and the signal is under that level, it will be limited by the threshold level.

I hope this helps you. If you still have questions or comments, please let me know.

Best regards,

Luis Fernando Rodríguez S.

Wow. Thanks for the explanation. You made me realize that your explanation and diagrams make sense only if that blue line is the signal amplitude of the ADC output into the left data channel and NOT the output of the PGA (which makes sense). This is because when you enable the noise threshold, the blue line gets lower BUT the output of the PGA stays the same. This means that the noise threshold feature only applies to the ADC gain and not the combined PGA and ADC gain. Can you confirm that is the intended operation of the silicon?

This then makes the noise threshold feature useless in analog bypass mode through the PGA. Can you confirm that? Please see diagram.

The Yellow signal is not attenuated or limited to the noise threshold. Only the Green signal has a gain of 0dB applied to it by only the ADC gain adjustment. The output to the headphones will never be affected by the noise threshold. Even when the Blue signal is grounded to 0V, the Yellow signal has high gain applied and very noisy even when noise threshold is enabled. 

The datasheet is not clear because it implies the combined ADC/PGA gain is adjusted:

The AGC Noise Threshold Flag is a read-only flag indicating that the input signal has levels lower
than the Noise Threshold, and thus is detected as noise (or silence). In such a condition the AGC
applies a gain of 0dB. <<< No, the AGC applies only an ADC gain of 0dB and keeps the PGA gain high

It is strange that the other features of the AGC apply to the PGA but the noise threshold does not. It would save a lot of time if the datasheet said this.

You Rock! Thanks so much for your help and please let me know if you think I'm mistaken.

Noland