TK Control, WDRC, and Expansion (How they are used and compared in fitting hearing aids)

Of topics that are confusing to many who fit hearing aids are the distinctions between the TK Control, WDRC, and Expansion. Understanding the differences between these and how they are used for providing appropriate amplification is a critical part of hearing aid selection and consumer satisfaction.

Basics

Basic definitions to help in understanding these differences:

1. Linear Amplification: 1:1 input/output ratio. 45 degree angle.

2. Compression: Less than 1:1 amplification. Less than 45 degree angle

3. Expansion: Opposite of Compression. Greater than 1:1 amplification. Greater than 45 degree angle. One way to control the potential for circuit noise or feedback in quiet environments. It is easier to impliment this action in digital than in analog.

4. WDRC (Wide Dynamic Range Compression) = input compression having a low knee point.

5. TK Control: One way to adjust the compression of a WDRC AGC aid to avoid circuit noise or to reduce feedback in quiet environments.

6. TILL: Trebel increases at low levels. {{1}} [[1]] Classifying Automatic Signal Processors, Killion, M., Preves, D., and Staab, W., Hearing Instruments, 1990; Vol. 41, No. 8, pp 24-26.[[1]]

WDRC Vs. Expansion

Study the two graphs in Figures 1 and 2. They show Expansion versus WDRC two different ways – first with respect to output, and second, with respect to gain.

Figure 1 shows the comparison in the more traditional way, The WDRC curve shows 40 dB gain (0 dB input and 40 dB output = 40 dB gain, as does a 20 dB input and 60 dB output, etc.) up to the knee, after which amplification becomes a 2:1 compression, or 1 dB out for each 2 dB input. So, the WDRC has the same amount of gain (40 dB in this case) from soft up to the knee. Very soft inputs are amplified to 40 dB. Under these circumstances, all soft sounds are made louder, including the mic noise (about 25 dB SPL). An additional problem occurs with WDRC in that in quiet a person may experience a hissing sound, or even feedback, but not when the signal input is stronger. Again, the reason for this is because the aid has maximum gain for soft sounds (40 dB gain in our example). In hearing aids that have a TILL processor (K-amp and almost every other aid that says it makes soft speech loud and loud sounds soft), the maximum gain is in the high frequencies, just where feedback is most likely to occur.

On the other hand, the expansion shows that at 20 dB input there is 40 dB output, or a gain of 20 dB. This is half the gain of the WDRC, and as a result, soft sounds are not amplified to the same extent as is the WDRC managed signal. At 40 dB input the output is 80 dB, which is now a 40 dB gain, the same at this point as the WDRC had from 0 dB input. So, you can see that expansion has its greatest amount of gain exactly at the knee point, and less gain for softer input signals.

Figure 1. Comparison of compression and expansion expressed in output.

Figure 2 is plotted as a result of the input to the aid. It shows the same as the above graph but is expressed in gain rather than output. The WDRC has constant gain of 40 dB up to the kneepoint.

Figure 2. Comparison of compression and expansion relative to gain.

For the example shown, the gain differences are:

 Input               WDRC                Expansion
0 dB              40 dB gain            0 dB gain
20 dB            40 dB gain          20 dB gain
40 dB            40 dB gain          40 dB gain
60 dB            30 dB gain          30 dB gain
Expansion functions well with digital aids because it can be implemented fairly easily via an algorithm.

TK (Threshold) Control

A way to adjust the AGC (compression) of an aid

This controls gain for soft input sounds only by adjusting the compression kneepoint over a ralatively low input level range (typically from about 40 to 55 dB).

As such, it is a gain booster for soft sounds. So, if there is too much circuit noise heard for soft sounds, move the kneepoint to a higher value, which will result in less gain for soft sounds. However, it will not affect the level of the MPO.