### Notice

Please note that most of the software linked on this forum is likely to be safe to use. If you are unsure, feel free to ask in the relevant topics, or send a private message to an administrator or moderator. To help curb the problems of false positives, or in the event that you do find actual malware, you can contribute through the article linked here.
Topic: Global gain, scale factors, signal, quantization (Read 4094 times)
0 Members and 1 Guest are viewing this topic.

## Global gain, scale factors, signal, quantization

##### 2003-08-26 11:01:47
This could belong to both mp3 and aac forums, so I am asking here.

My question is about how to quantize (and dequantize) the signal. I have some doubts regarding how factors are interacting with each others.

*global gain and scalefactors:
How is the scaling done?
Is the scaling done by (global gain * scalefactor) or by (global gain - scalefactor)?

*scaling:
What is scaled, signal or quantizer step size?

## Global gain, scale factors, signal, quantization

##### Reply #1 – 2003-08-26 11:40:47
Quote
*global gain and scalefactors:
How is the scaling done?
Is the scaling done by (global gain * scalefactor) or by (global gain - scalefactor)?

It should be:

actual_local_sf = global_gain - stored_local_sf;

Where - actual local sf would be an actual scaling factor for the particual band, and stored_local_sf would be a value stored in the encoded bitstream.

Quote
*scaling:
What is scaled, signal or quantizer step size?

During encoding, MDCT floating-point signal is scaled according to the quantizer step size, and the scaled representation is quantized (converted to integer domain).

## Global gain, scale factors, signal, quantization

##### Reply #2 – 2003-08-26 11:56:12
I think that I am still missing a part:

let's call ix the mdct output signal (floating point), do we quantize:
actual_local_sf * ix  ? (of course actual_local_sf is varying between sf bands)

## Global gain, scale factors, signal, quantization

##### Reply #3 – 2003-08-26 13:17:59
Quote
I think that I am still missing a part:

let's call ix the mdct output signal (floating point), do we quantize:
actual_local_sf * ix  ? (of course actual_local_sf is varying between sf bands)

do for every frequency coef w:
QuantSignal[w] = NINT( pow( ix[w] / actual_local_sf, 0.75 ) - 0.0946 )

Where:

int QuantSignal[w]  --> integerized, quantized value of spectrum
float ix[w] --> input MDCT signal, floating point
actual_local_sf --> global_gain - scale_factor[sfb]

## Global gain, scale factors, signal, quantization

##### Reply #4 – 2003-08-26 13:31:46
Thank you very much for those clarifications Ivan.

## Global gain, scale factors, signal, quantization

##### Reply #5 – 2003-08-26 14:07:53
Quote
QuantSignal[w] = NINT( pow( ix[w] / actual_local_sf, 0.75 ) - 0.0946 )

Ah, yes - I forgot

So, it can be seen that if you increase scale_factor[sb],  the actual_local_sf value will be decreased (actual_local_sf = global_gain - scale_factor[sb]), and the quantized signal will be amplified - and therefore closer to the original MDCT signal.

That's why in the loop iteration process, the distorted bands get amplified and re-checked for distortion.

## Global gain, scale factors, signal, quantization

##### Reply #6 – 2003-08-26 14:47:32
Language is strange sometimes.
It seems to me that by increasing the scale_factor (so decreasing the global scale value) the signal is not really amplified but less devided.
Overall it is equivalent, but still strange.

(I was asking those clarifications because I was wondering about the sfb21 strangeness)

## Global gain, scale factors, signal, quantization

##### Reply #7 – 2003-08-26 15:04:52
Quote
It seems to me that by increasing the scale_factor (so decreasing the global scale value) the signal is not really amplified but less devided.
Overall it is equivalent, but still strange.

Well, it is a power-law quantizer,  in reality it is really an amplification - and in the same time the deviation gets smaller