Sony ICD-ux560: Low Cut Filter (LCF) and Noise Cut Filter (NCF)

Sony ICD-ux560 comes with two recording filters; noise cut filter (NCF) and low cut filter (LCF). According to the ux560 manual, the NCF “cuts high-frequency sounds except for human voices as well as low-frequency sounds”. And the LCF, “cuts low-frequency sounds, including noise from projectors and roaring wind sounds.” That’s it.

I’ve tried to find more details about how these two filters work. I even reached out to Sony and they sent me back to the online manual. So, I set out to do some reverse engineering and figure out how the two filters work – like I did with the voice activation.

What was I looking for? First, the cut off frequency. Then, the slope of the filter attenuation. And with the ICD-ux560, it was not as straight forward as I thought. Let’s start with the less complex low cut filter (LCF).

Low Cut Filter (LCF)

This is also known as a high pass filter. And its function is to remove low frequencies from an audio signal. They are designed to remove frequencies below a certain determined frequency. This frequency is the cutoff frequency, sometimes referred to as the filter knee.  Normally, the cutoff frequency is set somewhere between 50 Hz and 150 Hz.

Past the cutoff frequency, the low cut filter has a slope, which means there is more and more attenuation (increase in removal of low frequencies) as the frequency gets lower. Typically, you’ll find that at and around the cut off frequency let’s say 100 Hz, you’ll get a 3dB reduction. At 50 Hz, you may find that you are getting a 6dB reduction in sound signal.

Now, this slope (or gradient) is usually rated as dB/octave. An octave is a halving of the frequency; 100 Hz to 50 Hz in our example. And our example filter would be rated as 3dB-6dB = -3dB/octave.

So, I set out to find the knee and the slope of the low cut filter on the Sony ICD-ux560. I set the recorder in a quiet room and recorded the room noise the LCF and the NCF. I then plotted the frequency graphs, which you can see in the image below.

[Image]

The red line is the frequency graph of the noise room sound without any filter. The blue line is the frequency graph using the low cut filter and the yellow line is the frequency graph while using the noise cut filter.

From the graph, we can make a few observations about on the low cut filter:

1. The cutoff frequency, or knee is at 280Hz, where you get about 3.6dB sound reduction. It does, however, continue to impact the sound well past the 280Hz frequency – till about 1.2kHz.
2. There’s another prominent knee at the 150Hz, when the slope is reduced from 6.9dB/octave to 5.4dB/octave. At the 150Hz mark, you are getting 8.6dB reduction in sound.
3. Finally, the slope does not change between 0-50Hz. You get a 17.8dB decrease in sound between these frequencies.

Noise Cut Filter (NCF)

From the Sony ICD-ux560 manual, I expected the NCF to be a combination of the LCF and a low pass filter. And this would create a frequency band that was optimal for voice recording.

Now, a low pass filter is very similar to a high pass filter (LCF), expect that it limits high frequencies. Low pass filters are great for establishing spatial contrast between two signals. For instance, if you are recording an interview, and there’s high frequency background noise, a low pass filter will contrast the two sounds and make your subject seem louder. Used in conjunction with a high pass filter, it greatly increases intelligibility.

Now, the Sony ux560 NCF uses a different high pass filter and combines it with a low pass filter. From observing the frequency graph of the noise cut filter:

1. The NCF kicks in at 20Hz. That’s earlier than LCF and you get 23.6dB sound reduction; 5dB more than LCF.
2. The NCF does have its knee at 150Hz, and the second at 280Hz. The slope for are 8.5dB/octave, and 6dB/octave respectively.
3.  At 280Hz, you get a 3dB sound reduction, that continues to impact the sound well past the 280Hz frequency – till about 1.2kHz.
4. The low pass filter, kicks in at around 3kHz and by 12kHz you’re getting about 9dB reduction.

Conclusions

The LCF and NCF recording filters on the Sony ICD-ux560 are complex; they have multiple cutoff frequencies and slopes. Resulting in something akin to dynamic processing. And surprising, the NCF has a more robust high pass filter than the LCF.

Should you use the LCF and NCF filters on the Sony ICD-ux560. If you’re recording voice, definitely recommend using the LCF. And if you are recording in a noisy location – try the NCF.

That’s it for this post. Hope you enjoyed it.  If you have any suggestions, comments or questions, feel free to post them in the comment section below. Happy recording.