Nyquist Teoremi - Örnekleme Frekansı (Sampling Frequency) İçindir

Nyquist Teoremi Nedir
Nyquist teoremine göre analog sinyalin dijital hale getirilmesi için analog sinyaldeki en yüksek frekansın genişliğinin/aralığının 2 katı kadar örnekleme yapmak gerekir. Burada genişlik/aralık kelimesi önemli. Eğer frekans aralığımız 9 - 12 kHz aralığındaysa, 6 kHz'de örnekleme yapmak gerekir. 12'nin 2 katında yani 24 kHz'de örnekleme yapılacağını düşünmek doğru değil. Çünkü aralığımız 3kHz.

Eğer analog ses sinyali saniyede 8000 defa örneklenirse ve her bir örnek 8 bit olursa bu da toplamda 8bit x 8000/s=64.000 bit/s eder.

Örnekleme Frekans Düşükse
Yüksek frekansları kaybederiz. 44kHZ ile örnekleme yapılmış bir kayıt 22 kHZ haline getirilirsin. Bu durumun açıklaması şöyle

the main problem with your sample is lost high frequencies due to resampling from 44kHz to 22kHz,

Örnek

Soru ve cevap şöyle

Q: We have an audio emitter that can emit two signals:

It either emits a sine wave at 23 kHz or it emits a sine wave at 25 kHz.

The receiver has the following sampling frequencies available: 16 kHz, 32 kHz and 48 kHz.

The question asks which is the best sampling frequency for the receiver to know when is 23 kHz being transmitted, and when is 25 kHz is.

A: When you sample at below the Nyquist rate, aliasing happens. That means frequencies higher than half the sampling rate get folded back down to below half the sampling rate.

Have a read about bandpass sampling.

PS: Tell your teacher, that's a really nice question. :-)

İnsanların Duyma Frekans Aralığı

İnsanın hava zerreleri aracılığıyla gelen sesleri duyma aralığı 20 Hz ile 20 kHz - yani 20,000 Hz - arasında değişiyor. 

Konuşma Frekans Aralığı

Açıklaması şöyle

Normal speech seldom has frequencies above 11 kHz

Ultrason (Ultrasound) Ses

Açıklaması şöyle. Bu sesi duymasak bile insan vücuduna zarar verebilir.

Ultrasound is used to break kidney stones and "burn" brain tumors. So no surprise that it can damage body parts, including inner ear. The effects of ultrasound on human body are multiple. One of them is heating due to absorption of the mechanical energy in the tissue and conversion to heat. This is used in physiotherapy as a desirable effect but it can be damaging if too much power is absorbed. There is also a mechanical effect. Parts reached by ultrasound vibrate so if the amplitude is too high they may break. The effect is used in ultrasound cleaners as well in medical field, to break apart stones and blood clots. Again, too much can do bad things. For this reason there are strict rules on maximum power allowed in any ultrasound imaging system. These effects have nothing to do with the fact that our ear can or cannot hear the sound. In similar way, your eyes can get burned by microwaves even though you don't see them.

Duyma Sinirleri

Açıklaması şöyle. 

Humans hear the correct perceptive signal for a sound wave of that frequency.

We really can't say much more than that. The psychology of acoustics are very complicated and could fill volumes.

It's closer to say we have cells which act resonant at a specific frequency. Our brain identifies which cells are resonating at any point in time, and constructs the signal from that. Our brains receive information that cell A or cell B is signaling. The association between those neural signals and frequencies is a learned response that we pick up early on, as an infant or perhaps even in the womb.

Nyquist Teoremi ve Görme

Nyquist teoreminin göz yani görme duyusu için uygulanamayacağı belirtiliyor.