TDA7514 datasheet(69/74 Pages) STMICROELECTRONICS

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TDA7514

The de-emphasis filter is shifted to a seven times higher corner frequency in AM mode with respect to the

FM mode.

5.4.9

AUDIO NOISE BLANKER SECTION

The operation of the audio noise blanker varies in FM and in AM.

5.4.10 FM MODE

The FM noise blanker triggering circuit acts as a peak-to-average detector on the high-passed MPX sig-

nal. The input 140 kHz high-pass filter removes the desired audio part so that the impulse noise is more

easily detected. The high-pass signal then follows two different paths:

– after rectification it is fed to one terminal of the trigger comparator; the impulse noise is present on

this path together with high frequency noise;

– after rectification it is fed to a slow peak detector which is not able to follow the impulse noise but

whose output (PEAK signal) represents the white high frequency noise level; the output of the slow

rectifier is the main input of the threshold generation circuit, whose output is applied to the second

terminal of the trigger comparator.

The threshold generation circuit generates a threshold as a monotonically increasing function of the PEAK

signal. The function can be programmed in its linear coefficient and in its second order coefficient.

For superior performance in the dynamically changing car-radio environment the activation threshold is

further influenced by three other parameters: field strength, FM frequency deviation and multipath pres-

ence.

The influence of these parameters can be disabled and is programmable. The parameter influence on the

noise blanker sensitivity is as follows:

– field strength: when the field strength decreases the noise blanker less becomes less sensitive (at

low field strength white noise becomes higher and false triggering becomes more likely);

– frequency deviation: if the FM frequency deviation is high, the noise blanker becomes less sensitive;

this is due to the fact that a large deviation causes a high MPX level which in turn might not be reject-

ed enough by the noise blanker detector input high-pass filter, thus causing false triggering;

– multipath: the presence of a strong multipath condition increases the sensitivity of the noise blanker.

The triggering comparator output activates a retriggerable monostable circuit whose output drives the

"Hold" switch in the high cut filter section. The blanking time is programmable and the whole noise blanker

action is defeatable via software.

5.4.11 AM MODE

In AM mode the noise blanker detector can operate in two different ways. For both modes the possibility

to low-pass the signal entering the noise blanker detector is foreseen (7.2 kHz LP filter software defeat-

able), in order to be able to reduce the white noise effect on the detector that may lead to false triggering,

especially for AM mode 1.

5.4.12 AM MODE 1

This noise blanker operation mode is similar to the FM operation mode. The input audio signal taken be-

fore the delay filter (see stereo decoder in AM) can be low-pass filtered (see AM mode description above)

and is subsequently high-pass filtered with a filter programmable in terms of corner frequency and order.

The resulting signal still contains the impulse noise information, high frequency noise (depending on the

activation of the 7.2 kHz filter) and audio (it is not possible to effectively eliminate all the audio content

because the AM channel bandwidth - determined by the IF2 ceramic filter - is barely wider than the signal

bandwidth, and the spectral differences between the impulse noise and the signal are small).

The signal is then applied to the same peak-to-average detector that is used for FM; the difference is that

the deviation detector is not influencing the threshold generation in this case. The noise blanking time, pro-

grammable also for AM, is about 30 times longer than for FM.

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