The function of de-emphasis circuit is -

Question

Prepp Team · Accepted Answer

Understanding De-emphasis Circuit Function

The question asks about the primary function of a de-emphasis circuit. To understand this, we first need to discuss its counterpart, the pre-emphasis circuit, as they work together in audio processing systems, commonly found in FM broadcasting.

Pre-emphasis and De-emphasis in Audio Systems

In communication systems like FM radio, the higher audio frequencies are more susceptible to noise. To improve the signal-to-noise (S/N) ratio for these higher frequencies, a technique called pre-emphasis is used at the transmitter side. Pre-emphasis involves boosting or amplifying the higher frequency components of the audio signal before modulation.

While pre-emphasis helps in improving the S/N ratio of high frequencies during transmission, it also alters the original frequency balance of the audio signal, making the high frequencies sound louder than they should be.

This is where the de-emphasis circuit comes in. The de-emphasis circuit is located in the receiver. Its function is to reverse the effect of pre-emphasis. It attenuates (reduces) the higher frequencies by the same amount they were boosted at the transmitter. This brings the high frequencies back to their original relative levels compared to the lower frequencies, restoring the natural tonal balance of the audio signal.

How De-emphasis Restores Original Frequency Levels

Think of it as a two-step process:

Step 1: Pre-emphasis (Transmitter): High frequencies are boosted. This makes them stronger and less vulnerable to noise pickup during transmission.
Step 2: De-emphasis (Receiver): High frequencies are attenuated (reduced) by the same amount they were boosted. This cancels out the pre-emphasis effect and restores the original frequency response.

By restoring the high frequencies to their original levels, the de-emphasis circuit ensures that the listener hears the audio signal with the correct frequency balance while benefiting from the improved S/N ratio provided by pre-emphasis.

Analyzing the Options for De-emphasis Function

Let's look at the provided options based on our understanding:

Option 1: To boost HF response of audio amplifier - This is the opposite of what a de-emphasis circuit does. Boosting high frequencies is the function of pre-emphasis or an equalizer used for boosting, not de-emphasis.
Option 2: To maintain adequate S/N ratio - While the *combination* of pre-emphasis and de-emphasis helps maintain or improve the overall S/N ratio, the de-emphasis circuit itself achieves this by reducing noise (which is more prevalent at high frequencies) along with the boosted high frequencies. However, its *primary* function is the restoration of the frequency response after pre-emphasis, which *indirectly* contributes to the perceived S/N ratio improvement without altering the original audio spectrum balance. Option 3 is a more direct description of its function.
Option 3: To bring back higher frequencies to their original level - This accurately describes the function of the de-emphasis circuit. It attenuates the boosted high frequencies to restore the audio signal's original frequency response.
Option 4: To bring back lower frequencies to their original level - Pre-emphasis and de-emphasis primarily affect the higher frequencies. Lower frequencies are generally less boosted (or not boosted) by pre-emphasis and thus do not require significant attenuation by de-emphasis to restore their original levels.

Circuit	Location	Function	Effect on High Frequencies (HF)
Pre-emphasis	Transmitter	Boosts HF to improve S/N ratio during transmission	Boosts HF level
De-emphasis	Receiver	Attenuates boosted HF to restore original frequency response	Reduces (attenuates) HF level

Therefore, the main function of a de-emphasis circuit is to compensate for the pre-emphasis applied at the transmitter, specifically by bringing the higher frequencies back to their original relative levels.

Revision Table: De-emphasis Circuit Key Points

Concept	Description
De-emphasis Purpose	Reverse the effect of pre-emphasis.
Action on HF	Attenuates (reduces) boosted higher frequencies.
Location	Receiver circuit.
Overall Benefit	Restores original audio frequency balance while leveraging S/N improvement from pre-emphasis.

Additional Information: Pre-emphasis and De-emphasis Constants

The degree of pre-emphasis and de-emphasis is defined by a time constant, typically set by an RC circuit. Common time constants are:

FM Broadcasting (USA): \( \tau = 75 \, \text{µs} \)
FM Broadcasting (Europe/Australia): \( \tau = 50 \, \text{µs} \)
Audio Recording (e.g., old tape formats): Different time constants were used (e.g., 3180, 318, 120 µs for NAB equalization).

The de-emphasis circuit has a frequency response that is the inverse of the pre-emphasis response, ensuring that when combined, they result in a flat overall frequency response for the audio signal, while the noise (which enters after pre-emphasis) is effectively reduced at higher frequencies.