Compression Flashcards
What is linear amplification?
A HA circuit that provides the same amount of gain at a particular frequency, for ALL input levels
Describe peak clipping
Used when output signal reaches a predetermined level (can coincide with ULL) and chops off the peak of the wave. It restricts:
Height
Intensity
Maximum output of HA
Will also reduce the level saturation occurs at
When would we use peak clipping? Type of HL and other circumstances.
Mild SN loss
Conductive loss
Mixed HL
simple lifestyle
Sudden loud sounds
What is the purpose of compression?
Ensure the audibility of soft sounds (higher level of gain)
Maintain recognition of moderate sounds (reduced level of gain)
Maintain comfort of loud sounds (little or no gain)
What are AGC and AVC? What do they do?
Automatic gain control
Automatic volume control
Both are non-linear amplification systems.
Work in the same way as linear HA’s until input reaches a certain level - knee point:
Less gain for loud inputs
More gain for soft inputs
2 commonly used AGC systems? And describe their purpose
AGCi - automatic gain control input
Input compression.
Considers INPUT level to determine when compression occurs, comes before volume therefore affects gain and output.
AGCo - automatic gain control output
Output compression
Considers OUTPUT level to determine when compression occurs. Comes after volume therefore affects only gain.
What is linear compression?
Where compression ratio is FIXED ie will always be a 2:1 CR whilst aids is in compression
What is curvilinear compression?
Opposite of linear, CR varies with input level
What is WDRC?
Wide dynamic range compression - uses whole of dynamic range using greater gain for soft sounds, mild gain for speech and no gain for loud.
CR must be below speech and have a low CR meaning aid nearly always in compression.
Low CT and low CR.
What are attack and release times?
ATTACK is the time taken for CT to be exceeded and circuit knows to go into compression. Should be short ie under 10 milliseconds
RELEASE is time taken for circuit to deactivate compression within 2dB of its linear gain once signal gone.
Speech intelligibility can be affected if too slow.
What is adaptive compression?
Release time depends on DURATION of LOUD input.
Sudden loud sounds activate a fast release.
With a continuous loud sound release is delayed until the sound is gone.
Compression adapts to duration and loudness of input.
What is dual compression?
Short release for high frequency sounds
Longer release for low frequency BGN
What is syllabic compression?
Uses fast attack and release times with duration of a syllable.
Compresses vocalised elements of speech (louder vowels) while allowing softer consonants to pass through.
What is multi channel compression?
The input signal is divided into frequency bands. Each channel can be adjusted to meet compression and gain needs of patient.
Therefore, gain and frequency will vary with intensity levels.
It is a LEVEL DEPENDENT FREQUENCY RESPONSE aid.
All previous compressions have varying gain but frequency is FIXED therefore they are FIXED FREQUENCY RESPONSE aids.
What is expansion?
It reduces the audibility for very soft sounds. Intend to prevent HA from amplifying soft sounds which are of no interest to the patient ie fridge or fan.
