Version: 1.18
PRELIMINARY/CONFIDENTIAL
TrueSpeech® Co-Processor
9.8.9
Reading the Playback Signal Level Value
It is possible to read the current average Playback signal level value using the command:
Command CF8FH:
1100 1111 1000 1
(15-3)
111
(2-0)
Status:
Average Playback Level Value
(15-0)
The level value is calculated using the following algorithm:
LEVEL = 0
FOR each of 240 OUTPUT samples per frame
IF OUTPUT < 0 THEN OUTPUT = -1 * OUTPUT
LEVEL = LEVEL + OUTPUT
LEVEL = LEVEL/256
Note: This is an average signal level (not r.m.s.). Division by 256 instead of 240 leads to a slight
understatement of the true average level. The fractional error is 240/256 = 0.9375, or 6.25%.
Dividing by 256 instead of 240 requires less processing by the CT8022 device.
When the CT8022 is operated with a base frame size that is not set to 240, the level calculation is adjusted to report
the same level as if the base frame size had been set to 240. However, since the CT8022 will update and report the
level for each frame processed, with a smaller base frame size the CT8022 will report the level measured over a
shorter period. This may result in more frame-by-frame variation in the signal level measured. When measuring the
level of a signal with lower frequency components, or with a low frequency modulation, more ripple may be
observed in the reported signal level. One example of this would be reporting the signal level of a 350+440 Hz dial
tone.
9.8.9.1
Level Scale
The level scale is relative to the maximum positive full scale value of 7FFFH (decimal 32767). This corresponds to
an input signal level of approximately + 3dBm0 from the external µ-law CODEC.
The precise input signal level scale can be calculated thus:
Output Level in dBm0 = +3.17 - 20 log(base 10) ( (output level/32767) * 256/240)
so the default threshold level of 0100H = 256 decimal, corresponds to 44.753 dBm0.
To calculate the actual average output signal level, read the LEVEL from the device and then perform the following
calculation:
CORRECTED LEVEL = (LEVEL * 256)/240
output level in dBm0 = 3.17 - 20 * log10( CORRECTED LEVEL/32767)
CT8022A11AQC FW Revision 0118 DSP GROUP, INC., 3120 SCOTT BOULEVARD
SANTA CLARA, CA 95054 PH: 408 986 – 4300 FAX: 408 986 – 4490
83
All specifications are subject to change without prior notice.
ETC [ ETC ]
