![]() ![]() For example, if speech is all above 150 Hz, then notching out hum at 60 and 120 Hz is not going to make this speech ring. If the useful signal does not share frequencies with the notches, then ringing is not going to happen. In other words, the filter has to “cut through the signal”. ![]() ![]() The narrower the notches, the longer the ringing.ĭoes ringing always happen with notch filters? One condition has to be met for it to happen: transient signal energy must happen at the same frequency as the notch (or, more generally, - at any bends in the frequency response). In effect, the signal itself begins to ring. Is ringing bad? When a signal transient passes through the filter, the ringing of the filter’s impulse response is imparted on the signal through a mathematical operation called convolution. It is soft and subtle, so headphones are recommended. Take a listen to the click of the impulse as the Q is increased. This ringing matches in frequency with the bends in the filter’s frequency response. These tails are called ringing because they add a reverberant, ringing character to the perfect click of the impulse. More detailed and jagged frequency responses inevitably require more spreading of the impulse response in the time domain.įor example, increasing the Q of a notch filter makes it narrower, but adds more “tails” to the impulse response of the filter. One important fact about filters is that frequency response and impulse response are related. The impulse response determines what the filter does to the signal waveform and how it handles transients. The frequency response determines how the filter is changing amplitudes of different frequency components in the signal (as in the plot above). What if we make the notches narrower by increasing the Q? This also has an important side effect called ringing, which can potentially damage the signal more than the original hum does.Įvery notch filter (and every EQ in general) can be characterized by its frequency responseand its impulse response. When notch filters are stacked together, their frequency responses are also adding up and attenuating more of the useful signal, like in the picture below. quality factor which defines how narrow is the notch). Every notch filter has a frequency, attenuation depth, and Q (a.k.a. Would it be practical to use hundreds of notch filters? Possibly, but one has to consider the side effects. They sound more like buzz and can extend beyond 1 kHz or even 10 kHz and contain hundreds of harmonics. ![]() However some variants of hum are more wideband. Is this enough? Many real-world hums are primarily concentrated at lower frequencies. For typical fundamental frequencies of 50 or 60 Hz, this covers the range up to roughly 1 kHz. In RX it was capped at 8 for a while, until RX 9 extended it to 16. One obvious limitation is the number of harmonics one can attenuate. Sounds good on paper, right? Well, it is time to learn about the limitations of this approach. What is the simplest way to kill an unwanted tone? Every engineer will say: a notch filter! So, the traditional design of a De-hum module was based on a battery of notch filters tuned to harmonics of a base tone, such as 60 Hz. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |