Amplifiers and Compressors Flashcards
What are the three different types of amplifiers?
Field effect transistor (pre-amp)
Transistor based amplifiers
Output amplifier (final amplifier)
What is the purpose of the field effect transistor?
To increase the amplitude of the initial analog electric signal, making it more robust as it progresses through other components
What is the purpose of transistor based amplifiers?
To add gain to the input signal, increasing the amplitude of the electrical signal
Occurs before volume control
What is the purpose of the output amplifier?
Drives the signal to achieve the highest possible output
Happens after volume control
What is an input-output curve?
A chart showing the intensity of the input signal as it compares to the output SPL measured at the TM
What is the challenge with reduced dynamic ranges?
Maximizing the amount of sound fitting into the range
What is linear amplification?
Adding the same amount of gain at all input levels
1:1 ratio
Will linear amplification restore normal loudness growth?
No
It will under-amplify soft sounds and overamplify loud sounds
What is a compressor?
A level detector that will automatically adjust the amount of gain an amplifier adds to the input signal
T/F: The louder the signal, the less gain that is added
True
What is another term for compression?
Automatic gain control (AGC)
What are the two types of compressors?
Output compression (AGC-o)
Input compression (AGC-i)
What is output compression?
A level detector that compresses exclusively loud amplitudes after amplification
What is input compression?
A level detector at the pre-amplifier
Acts on incoming signals to sustain a constant output amplitude with an individual’s dynamic range
Assesses loudness of the incoming signal after it is changed to electrical
What are the 4 controls that affect compression?
Threshold kneepoint
Attack time
Release time
Compression ratio
What does the compression ratio determine?
The amount of gain added to the input signal
What is the threshold kneepoint?
The input level that activates the compressor
What is attack time?
The time it takes for a compressor to activate and automatically apply compression when the level crosses a TK
What is release time?
The time it takes for the compressor to deactivate when the level falls below the TK
Does a compressors attack time result in an overshoot?
Yes, because gain cannot change immediately
There is a period of over amplification
Fast acting = less time of overshoot
Does a compressors release time result in an output undershoot?
Yes, because it cannot act immediately
There is a period of under amplification
Does compression speed vary by input level?
Yes
Large sudden changes of input levels in the environment need fast active compression AT/RT
Small softer input level changes need slower compression AT/RT
What is a pro of fast acting compression?
It quickly adapts to new input levels
What is a con of fast acting compression?
When it is applied to soft signals, it compresses peak amplitudes of the spectral envelope and significantly changes it’s shape
Envelope no longer matches an individual’s auditory memory of a sound
Requires more working memory and listening effort
What is a pro of slow compression?
Preserves the original spectral envelope’s shape
Promotes naturalness of sound
Decreases listening effort
Preserves ILD between ears
What is a con of slow compression?
Soft sounds produced right after loud sounds may be inaudible
What kind of compression was used before output compression?
Peak clipping
Abruptly stopped adding gain to avoid over amplification of an output signal
Looks like a plateau
Turns into a square wave
What is MPO?
Maximum power output
The loudest output a device can produce regardless of the input level
What does the term saturation refer to?
The signal’s intensity exceeded the capabilities of a circuit
What does peak clipping result in?
A distorted output signal
Harmonic distortion
What does output limiting compression do instead of peak clipping?
Soft clipping
Maintained the sine wave shape, and reduced distortion
What is the compression ratio for output limiting compression?
5:1 to 10:1
What signals is OLC applied to?
Loud input signals
80-90 dB or louder
Does OLC still result in distortion?
Yes, but it’s less noticeable than peak clipping
What is considered a high compression ratio?
Ratios above 5:1
Does OLC restore audibility of soft input signals or normal loudness perception?
No
What is wide dynamic range compression?
It applies a low compression ratio to soft and moderately loud signals to improve audibility and restore loudness perception
AGC-i
In WDRC, is more gain added to soft signals and less is added to moderate?
Yes
What are the two variations of AGC-i that were introduced?
Bass increase at low level circuit (BILL)
Treble increase at low level circuit (TILL)
What is BILL?
Low frequency amplification cut after 50 dB
Alleviate the loudness of noise (usually below 1500 Hz) and upward spread of masking
What is TILL?
High frequency gain is boosted in quiet environments
Improves speech intelligibility
Between BILL and TILL, what is the most commonly used today?
Till
K-amp is the most common TILL circuit
What is the theory behind WDRC?
Cochlear damage results in a loss of sensitivity for soft sounds, with little or no loss of sensitivity for loud sounds
Soft inputs require more gain than intense sounds
The frequency response should change with varying input levels
What compression ratios are used for WDRC?
Low compression ratios
1.1:1 to 4:1
When do the compressors activate for WDRC?
At a soft input threshold kneepoint
20 to 50 dB
What is the purpose of WDRC?
To expand the dynamic range by allowing more access to sound before UCL is reached
Is it true that the lower the CR is, the less distortion will be in the signal?
Yes
Does shifting to a lower TK increase output for soft signals at or below the TK?
Yes
Does shifting the TK higher decrease the output for soft signals?
Yes
What is the purpose of expansion?
To manage the intensity of very, very soft input signals and circuit noise
What is the purpose of using a linear compression ratio below the first TK?
To attenuate very soft input signals
How does expansion work?
As the input of the signal increases, the amount of gain is increasing too
Opposite of WDRC and OLC
What are the compression ratios for expansion?
Below 1:1
0.7:1 and 0.5:1 commonly used
Do contemporary hearing aids contain many more TKs than 2 or 3?
Yes
Curvilinear compression has so many TKs that it looks like a smooth curve
Do contemporary hearing aids have both AGC-i and AGC-o?
Yes
AGC-i to manage incoming sounds and AGC-o to manage loud sounds
When volume control is increased, what happens to the TK and the CR?
When the volume is raised, the TK shifts lower and the CR increases
Used so comfort doesn’t have to be sacrificed when raising the volume
Is AGC-i situated before volume control?
Yes, so changing the volume will have no impact on this TK or CR
Entire MPO increases as the volume is raised
Does abnormal loudness growth differ by frequency?
Yes, so I/O needs change based on the frequency too
What are frequency shaping bands?
Volume control separated into individual frequency ranges
Adds gain to a narrow range of frequencies based on the patient’s thresholds
Do changes to the frequency ranges change the CR?
No
Gain for all input signals increase equally
What are compression shaping channels?
Channels that take loud sounds and squeeze them into a dynamic range
Alter the output for sound levels at all or select frequencies
What are all of the elements of compression that vary by frequency?
Expansion
TK
WDRC CR
OLC CR
