The output signal-to-noise ratio (SNR) of a communication system is an important measure of its performance. It indicates the quality of the received signal relative to the noise present in the system. The higher the SNR, the better the quality of the received signal.

Modulation and demodulation are key processes in a communication system. Modulation refers to the process of encoding information onto a carrier signal, while demodulation is the process of extracting the original information from the modulated carrier signal. Both of these processes have a significant impact on the SNR of the system.

The output SNR depends on the type of modulation used at the transmitter and the type of demodulation used at the receiver. This is because different modulation schemes and demodulation techniques have different levels of noise immunity and efficiency.

Here is a detailed explanation of why the output SNR depends on the modulation and demodulation techniques:

1. Modulation Techniques:
- Different modulation techniques have different levels of noise immunity. For example, amplitude modulation (AM) is more susceptible to noise compared to frequency modulation (FM) or phase modulation (PM). This is because AM modulates the amplitude of the carrier signal, and any noise added to the signal affects the amplitude directly.
- The choice of modulation scheme affects the bandwidth utilization and spectral efficiency. Some modulation schemes allow for more efficient use of the available bandwidth, which can improve the SNR.
- The modulation index or modulation depth also affects the SNR. For example, in FM modulation, a higher modulation index results in a higher SNR because it provides better noise immunity.

2. Demodulation Techniques:
- Different demodulation techniques have different levels of noise rejection and recovery of the original signal. For example, coherent demodulation techniques, such as coherent detection or synchronous detection, provide better noise rejection compared to non-coherent techniques like envelope detection.
- The demodulation process should ideally recover the original signal without introducing additional noise or distortion. The efficiency and accuracy of the demodulation process affect the SNR of the system.
- Some demodulation techniques, such as differential demodulation, can provide better noise rejection and error correction capabilities, leading to an improved SNR.

In summary, the output SNR of a communication system is influenced by the modulation technique used at the transmitter and the demodulation technique used at the receiver. The choice of modulation and demodulation schemes determines the noise immunity, spectral efficiency, and accuracy of signal recovery, all of which directly impact the SNR of the system.