Masking

The Nugget

Audio masking is a covert technique used to hide intercepted audio, such as human speech, from both casual and professional interceptors. It employs various modulation schemes to make detection and demodulation difficult for surveillance receivers.

Make it stick

📡 Subcarrier modulation allows for audio transmission above audibility, disguising the signal for interceptors.
🔇 Double Sideband with Suppressed Carrier (DSBSC) eliminates the subcarrier to conceal the audio within its noise, making it harder to detect.
📊 Triple Pulse (TP) uses randomized pulse positions to obscure audio, leading to a dispersed signal that is difficult to track.
🔍 Super Pulse (SP) minimizes power usage and increases stealth by masking signals with noise during non-active periods.

Subcarrier (SC) Modulation
- Modulates audio onto a higher frequency subcarrier for transmission, favored by intelligence agencies like the CIA during the Cold War.
- Common frequencies include 12.5 kHz, 22 kHz, and 40 kHz.
Double Sideband with Suppressed Carrier (DSBSC)
- The audio signal is amplitude modulated onto a high-frequency carrier, which is then removed, avoiding detection by common bug finding equipment.
Triple Pulse (TP) Technique
- Samples audio at random intervals, encoding information within the pulse position, making it extremely difficult to demodulate.
Rejected Pulse (RP) Technique
- Similar to TP but intentionally drops certain pulses, creating irregularities that carry intelligence, recoverable only with specific compatible receivers.
Dirty Pulse (DP) Technique
- Samples audio with variable positioning of pulse segments to mask information; proved to be robust against detection methods.
Super Pulse (SP) Method
- Evolved from DP to enhance stealth by masking signal transmissions with noise when not in active use.

Advanced receivers like the Scanlock series can identify SC signals, while PR-100 can locate various signals effectively.
Conventional surveillance tools struggle against PPM-based signals, requiring specialized manual searching techniques.

Techniques like SC and DSBSC have roots in the CIA's early surveillance efforts, reflecting a constant evolution in audio masking for espionage.
Notable devices used include CIA's SRT series equipped with these masking methods, highlighting ongoing advances and military applications in covert operations.

"Audio masking is critical for maintaining intelligibility in covert listening devices while evading detection."
"Professionals often rely on complex modulation schemes to obscure the audio intelligence from potential interceptors."
"For every bug that has been found, there are four undiscovered ones."
"Subcarrier modulation was first employed in wired telephony to increase network capacity."
"The TP masking scheme was developed for long-term use but found itself superseded by more reliable techniques."

This summary contains AI-generated information and may have important inaccuracies or omissions.