Audioblogger.com QoS T&'M

Purpose
Determine the quality of service (QoS) of the Audioblogger.com system by performing some test and measurement (T&M) on the signal channel.

Procedure
Input a known test signal over a GSM cell phone to the Audioblogger system and then perform distortion analysis on the captured data file. Constant sine wave, linear sine sweep, and white Gaussian noise test signals were created by baudline's function generator. These test signals were played back by a CS4236B codec to Altec Lansing ACS48 speakers and then to the headset microphone of a Motorola v60 GSM cell phone. The captured Audioblogger file was a 24 kbps MP3 audio file with a 22050 sample rate. Click below to listen:

This is a complicated signal path that has many potential points of degradation. It is difficult to guess which step is the weakest link. The signal path consists of:

1. baudline plays test signal
   
2. CS4236B DAC (44100 sample rate)
   
3. Altec Lansing ACS48 speakers
   
4. headset microphone
   
5. Motorola v60 GSM cell phone (4 bars)
   
6. carrier's GSM to u-law conversion
   
7. Audioblogger machine's ADC (22050 sample rate)
   
8. MP3 24 kbps compression
   

Analysis
Below is the spectrogram of the entire file. Notice that it is VGC limited at about 4kHz of bandwidth.



The test signal consists of three different test stimuli. From top to bottom. The first section is a constant sine. The second section has three copies of a linear sine sweep. The third and final section is white Gaussian noise (WGN). In between the tests are sections of silence which are used to measure the idle noise floor.

Below is a zoomed timebase version of the first sine wave section.



It looks like an AGC kicks in after about 3 seconds of steady state signal. This isn't good. Signal gain drops by 5 dB and a strong 100 Hz tone appears, then in the final 8 seconds the sine wave looks fizzled out.

The 2nd linear sine sweep was the cleanest and the zoomed in image is below. The 1st and 3rd sweeps were discarded because they have strong clipping distortion near the end that look like wideband noise. In all three runs notice the crisscross harmonics and the aliasing around the 4kHz Nyquist point.



The same baudline DAQ / codec copy-n-paste technique (see references) was used to visualize the individual spectral parts. In the Average window the following colors correspond to the different sections:

• green - 301.99 Hz sine wave
  
• purple - noise floor (silence)
  
• orange - white Gaussian noise (WGN)
  
• cyan - sine sweep 0 - 4000 Hz
  

The purple noise floor and orange WGN have subtle peaks at ~460 Hz. The green sine wave has a large number of distortions. The spectrum of the cyan sine sweep is lumpy and resonances are visible. Here are the distortion measurements from the first 2.5 seconds of the sine wave. The numbers drop significantly after that (by about 1.8 bits).

Distortion Metrics:

• frequency: 301.995 Hz (+16.56 PPM)
  
• SNR: +35.16 dB
  
• THD: -40.02 dB (0.01 %)
  
• SINAD: +33.93 dB
  
• ENOB: +5.344 bits
  
• SFDR: +41.78 dB
  

Here are some comparison ENOB's:

• CS4236B DAC (11.5 bits)
  
• GSM 6.10 (8 bits)
  
• u-law (6 bits)
  
• 24 kbps MP3 (15 bits)
  

Conclusion
All considered, the final ENOB of 5.3 bits is quite good. The Audioblogger system looks like it will work well for voice. The performance for field recordings is questionable but if all you have is a cell phone then Audioblogger might just be a useful tool.

References
http://www.baudline.com/solutions/full_duplex/
http://www.baudline.com/solutions/codec/