TDA1546T Datasheet, PDF (25/40 Pages) NXP Semiconductors – Bitstream Continuous Calibration DAC with digital sound processing BCC-DAC

Philips Semiconductors
Bitstream Continuous Calibration DAC with
digital sound processing (BCC-DAC)
Preliminary specification
TDA1546T
7.9.2 dB CONVERTER
Before peak data is output, the detected value is converted
from linear to a dB scale internally by the TDA1546T. This
has the following advantages:
• ease calculation load on the system microcontroller
• optimal use of dynamic range of the readout
• facilitate manipulation of sound processing control
levels in combination with peak readout levels, to allow
for e.g. dynamic tone control.
The internal linear peak detection occurs with a resolution
of 16 bits on the incoming left and right audio samples
individually. This linearly acquired value is converted to
dB's as shown in Table 14. Because of quantization of the
linear code, accuracy is lower for the very lowest detected
peak values. Some values in the lower range of the dB
scale have no counterpart in the linear scale, consequently
these values never occur as output peak words. This is
also illustrated in Table 14). The dB conversion block
converts only positive linear values to a a useful dB value.
All negative input values will be converted to an output
value of 3 for recognition.
Table 14 Peak readout linear-to-dB conversion (note 1)
PEAK
DATA
000000
000100
001000
001100
010000
..........
PEAK VALUE
(dB)
−∞
n.a.
n.a.
n.a.
−71.22
............
PEAK
DATA
000001
000101
001001
001101
010001
111101
PEAK VALUE
(dB)
n.a.
n.a.
n.a.
−76.33
−69.48
−2.87
PEAK
DATA
000010
000110
001010
001110
...........
111110
PEAK VALUE
(dB)
n.a.
n.a.
−80.77
−74.75
..........
−1.48
PEAK
DATA
000011
000111
001011
001111
...........
111111
PEAK VALUE
(dB)
−90.31
−84.29
−78.27
−72.25
...........
0.00
Note
1. The peak level dB conversion block relates according to the following transfer formula from linear to dB scale:
a) Peak value (dB) = (peak data − 63.5) × 5 × log 2
b) The table should be read as follows. The maximum value of 63 (111111) is returned when the detected value
resides between −1.48 dB and 0 dB, the next lower value of 62 is returned when the detected value resides
between −2.87 dB and −1.48 dB etc. Only true digital silence will return a peak readout value of 000000.
c) For peak data > 010011 (= 19) the error in peak level is <(11 × log 2)/4
d) For peak data <010100 (= 20) the error will be larger due to 16 bits accuracy.
7.9.3 PEAK DETECTION
The TDA1546T provides a convenient way to monitor the
peak value of the audio data, for left and right channels
individually, by way of readout via the microcontroller
interface. Peak value monitoring has its applications
mainly in digital volume unit measurement and display,
and in automatic recording level control. The peak level
measurement of the TDA1546T occurs with a resolution of
16 bits thus providing a dynamic range amply suitable for
all practical applications.
The output of the peak detection block is a register of two
6-bit words (one for each channel) which represents the
dB linear value of the positive peak value and is accessible
via the microcontroller interface. The peak detection block
continuously monitors the audio information arriving from
the spectrum analyzer, comparing its actual dB value to
the value currently stored in the peak register. Any new
value greater than the currently held peak value will cause
the register to assume the new, greater value. On a peak
request (see Section 7.3) the contents of the peak register
are transferred to the microcontroller interface. After a
read action the peak register will be reset and the
collection of new peak data started. The end of a peak
read action should be marked by an address mode
sequence so that the output peak register is able to latch
new data.
The peak detection block receives data that has been
processed by the de-emphasis block, so if peak data is
read when applying digital de-emphasis, the de-emphasis
frequency characteristic will be noticeable in the peak
output value.
January 1995
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