# DC filtering for digital audio amplifiers

I am working with digital amplifiers and from what I can see there is a DC blocking filter that is placed at the start of the signal chain.

1. Why is this DC filter placed at the start of the signal chain?
2. The cutoff frequency of this filter is just 4Hz. Any intuition behind having a cut off as low as 4Hz?
3. This filter is a double precision filter. Is there any reason why DC blocking should be done with a double precision filter rather than a single precision filter?
• can you link to the data sheet or app note? Usually DC filters are there to ... block DC. We don't want DC in an audio chain. 4Hz is a bit low, 10-20 more common depending on the application. welcome by the way. – danmcb Apr 14 at 7:37
• Please add more info, is this filter you speak of analog, or digital DSP filter since you talk about single and double precision? – Justme Apr 14 at 7:43
• Electrodynamic speakers are the most common audio loads, and they don't get along very well with DC. The low frequency just sets the cut-off for the lowest audible frequency. As for precision, that's a matter of how much dynamic range you need. With float you can get a theoretical 120 dB, but even 100 dB should be fine (unless you're after world records). – a concerned citizen Apr 14 at 7:46
• This happens to read like a thinly-disguised homework question. – TonyM Apr 14 at 8:14
• Whether or not it's a homework or classwork question ,a question is a question and should be answered if you can ... Please avoid unnecessary rude comments or slighting remarks .... – Clauvunate Apr 15 at 4:38

## 1 Answer

1. DC is not audio and should not be amplified or be present at speakers as DC current can burn the speaker coil. Unwanted DC can also cause problems in the signal processing blocks, be them analog or digital, so the best thing to do is to just filter out out before processing the signal any further. Sometimes a processing block might require a certain DC level to operate properly, so of course the old DC bias must be filtered out before applying a new DC bias, this is done between analog portions in an amplifier and sometimes a small DC bias is added to a digital signal to avoid denormal numbers in processing.

2. The frequency is arbitrary. It must be high enough to block unwanted DC and drift, but low enough to pass audio with little phase distortion. Basically a rule of thumb is that for a simple analog RC filter, to pass audio down to 20 Hz with little phase change, the RC cutoff frequency needs to be a decade lower so 2 Hz. So a higher order 4 Hz digital filter can be good so that it is good enough to block DC and good enough to pass audio at maybe 20 Hz so the transition band is sharp enough with small amount of processing power.

Why a filter uses double precision arithmetic instead of single precision depends on what kind of filter it is, FIR, IIR, biquad, etc. Some constructs like biquads are very useful but need careful handling not to become unstable due to coefficient truncation etc. And another thing to remember is that single precision floating point arithmetic only gives 23 bits of mantissa, so in some sense it's a bit worse (pun intended) than 24-bit linear PCM, and that can make a difference.

• Well for the second part as to why the 4Hz choice of frequency I'm sorry I couldn't understand your explanation . – Clauvunate Apr 15 at 4:35
• Yes we can only guess why it is exactly 4, but it could be anything like 2 or 5 or 10 and we would still have to guess why. It is just a value that needs to be high enough to block DC fast enough, it needs to be low enough so it does not remove or distort useful audio content, and does not need too much hardware resources. – Justme Apr 15 at 5:58