Dear Experts,
My environment: DM8168/SDK Version: VCSDK(based on DVRRDK 2.8).
I have a question about AAC encoder (AAC_LC_Encoder_C674x).
Here is my original audio input spectrum
After encode this audio to aac, I got below spectrum
There are two gaps around 10kHz and 12kHz.
Is this a common characteristic of TI's AAC encorder?
By the way, I perform some experimental. Here is a attached file that record my result: FFT of dark noise_0223.docx
Since this cutoff happens on the range 10KHz~15.5KHz. So, I use the audio precision to generate 5.5KHz, 11KHz, 13KHz to observe the spectrum. The attached is my test result.
My test method is to change line input voltage from 0dBV to -90dBV to see if each single tone have the same amplitude in spectrum. The purpose is to check if there is any attenuation in the specific frequency by AAC process.
In attached file, the Figure 1 and Figure 2, the amplitude of 13KHz single tone is same as to 5.5KHz singe tone when I generate -20dBV and -40dBV signal. In the Figure 3, 13KHz single tone disappear when I generate -75dBV signal. It seems that AAC will drop the very low voltage signal between 12KHz~15.5KHz.
In the Figure 5, 5.5KHz and 11KHz disappear in spectrum since -90dBV signal is close to noise level.
According to this experiment, we think this strange cutoff only happens when input voltage is smaller than -75dBV. -75dBV input voltage is small, it is hardly heard by ear. Perhaps AAC drops the low voltage signal in high frequency for the compression.
Is this conclusion correct??
thx ~
HB
