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Microphone Audio Compressor in SSB Radio

Microphone Audio Compressor in SSB Radio

    Single-sideband (SSB) radio, a dominant mode in amateur (ham) radio and other communication applications, transmits voice signals with improved spectral efficiency compared to older AM (amplitude modulation) methods. However, SSB has limitations in terms of audio quality and intelligibility, particularly in challenging propagation conditions. This is where microphone audio compressors play a crucial role in improving the effectiveness of SSB communication.

Understanding SSB and its Audio Challenges

    SSB achieves spectral efficiency by transmitting only one sideband (upper or lower) of the modulated carrier signal. This reduces bandwidth requirements compared to AM, allowing more channels to be accommodated within a limited spectrum. However, SSB inherently compresses the audio dynamic range.

     Dynamic range refers to the difference between the loudest and softest parts of an audio signal. Human speech naturally exhibits a wide dynamic range, with whispers being significantly quieter than shouts. In SSB transmission without compression, quiet passages can be lost in background noise, while loud passages can cause excessive power consumption and potential signal distortion. This leads to inconsistent intelligibility for the receiving station.

     Another challenge with SSB audio is the presence of atmospheric noise, power line hum, and other interference. These unwanted signals further mask the desired voice communication, especially when the speech level is low.

The Role of Microphone Audio Compression

     A microphone audio compressor addresses these challenges by dynamically adjusting the gain of the audio signal before it enters the SSB modulator. It essentially “squeezes” the dynamic range, amplifying quiet passages and attenuating loud ones. This results in a more consistent audio level that is less susceptible to noise and distortion.

Here’s a breakdown of how a microphone audio compressor works:

  • Threshold: This sets the minimum level at which the compressor starts to act. Signals below the threshold remain uncompressed.
  • Compression Ratio: This defines the amount by which the signal exceeding the threshold is attenuated. A ratio of 2:1 means a 2 dB increase in the input signal will only result in a 1 dB increase in the output. Higher ratios lead to more aggressive compression.
  • Attack Time: This determines how quickly the compressor reacts to a sudden increase in signal level. A faster attack time controls peaks more effectively, but can introduce undesirable artifacts.
  • Release Time: This controls how quickly the compressor returns to its original gain state after a signal level drops below the threshold. A slower release time allows the audio to breathe more naturally, but may cause pumping effects where the gain seems to fluctuate.
Benefits of Using Microphone Audio Compression in SSB

    The primary benefit of using a microphone audio compressor in SSB is improved intelligibility, especially in noisy propagation conditions. By maintaining a more consistent audio level, the desired voice signal remains prominent amidst background noise, making it easier for the receiving station to understand.

Here are some additional advantages:

  • Increased Average Power: With compression, the peak-to-average ratio of the audio signal is reduced. This allows for a higher average power level within the limited SSB bandwidth, potentially improving signal strength and range.
  • Reduced Spurious Emissions: Excessive audio peaks can contribute to unwanted out-of-band emissions that can interfere with other radio signals. Compression helps mitigate this issue.
  • Reduced Operator Fatigue: By maintaining a consistent audio level, a compressor reduces the need for the operator to constantly adjust microphone gain, leading to less fatigue during extended QSOs (conversations).
Considerations for Using Microphone Audio Compression in SSB

    While microphone audio compression offers significant benefits, it’s crucial to use it judiciously. Here are some factors to consider:

  • Over-compression: Excessive compression can lead to a loss of natural dynamics and a “processed” sound that can be unpleasant to listen to. A good balance needs to be achieved between intelligibility and natural audio quality.
  • Pre-emphasis and De-emphasis: Some SSB radios utilize pre-emphasis and de-emphasis filters to improve high-frequency response during transmission and reception, respectively. These filters need to be taken into account when setting compressor parameters to avoid unwanted effects.
  • Different Operating Modes: SSB can be used for various purposes, such as casual QSOs, contesting, and DXing (long-distance communication). The optimal compressor settings may vary depending on the specific application. For example, contesting often favors more aggressive compression for improved signal strength, while casual QSOs might prioritize a more natural audio experience.
Implementing Microphone Audio Compression in SSB

There are two main approaches to implementing microphone audio compression in SSB:

  • Internal Compressor: Many modern SSB transceivers come with built-in audio compressors. These offer the advantage of convenient integration with the radio’s controls. They typically provide basic adjustments for threshold, compression ratio, and attack/release times.
  • External Compressor: For more advanced control and customization, external audio compressors can be used.

For more info:

Amateur Radio – Mike Preamplifier/Speech compressors/Audio Processors – Resources

LM324 microphone speech compressor

Dynamic speech compressor with audio preamplifier

Microphone compressor using LM1458

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