查看“︁脉冲密度调制”︁的源代码

{{noteTA
| G1 = Signals and Systems
| G2 = Communication
| G3 = Music
| G4 = IT
}}

{{Modulation techniques}}
'''脈衝密度調變'''（{{lang-en|'''Pulse Density Modulation'''，'''PDM'''}}）是一種[[二進制]]信號的[[模擬信號]][[調製]]方法。在PDM信號中，脈衝的相對密度對應模擬信號的[[幅度]]，並不像[[脈衝編碼調製]]（PCM）中那樣，特定的幅度值不被[[編碼]]成不同[[權重]]的脈衝碼。1位DAC的輸出與信號的PDM編碼相同。[[脈衝寬度調製]]（PWM）是PDM的一種特殊情況，其中開關頻率是固定的，並且對應於一個樣本的所有脈沖在數字信號中是連續的。對於[[分辨率]]為8位的50％[[電壓]]，PWM波形將打開128個時鐘週期，然後在剩餘的128個週期內關閉。利用PDM和相同的時鐘速率，信號將在每隔一個週期的開啟和關閉之間交替。兩種波形的平均值均為50％，但PDM信號更頻繁地切換。對於100％或0％的水平，它們是相同的<ref>Thomas Kite. [http://users.ece.utexas.edu/~bevans/courses/rtdsp/lectures/10_Data_Conversion/AP_Understanding_PDM_Digital_Audio.pdf "Understanding PDM Digital Audio" (PDF)] {{Wayback|url=http://users.ece.utexas.edu/~bevans/courses/rtdsp/lectures/10_Data_Conversion/AP_Understanding_PDM_Digital_Audio.pdf |date=20210225214155 }}. 2012. The "PDM Microphones" section on p. 6.</ref><ref>Maxim Integrated. [http://datasheets.maximintegrated.com/en/ds/MAX98356.pdf "PDM Input Class D Audio Power Amplifier" (PDF)] {{Wayback|url=http://datasheets.maximintegrated.com/en/ds/MAX98356.pdf |date=20210116064255 }}. 2013. Figure 1 on p. 5; and the "Digital Audio Interface" section on p. 13.</ref><ref>Akustica. [http://www.akustica.com/Files/Admin/PDFs/Datasheets/DS25-2.1%20AKU230%20Datasheet.pdf "AKU230 Digital, CMOS MEMS Microphone" (PDF)] {{Wayback|url=http://www.akustica.com/Files/Admin/PDFs/Datasheets/DS25-2.1%20AKU230%20Datasheet.pdf |date=20160404205632 }}. 2012. p. 5.</ref>。

==描述==
在脈衝密度調製的[[位元流|比特流]]中，1對應於正脈衝（+ A），0對應於負脈衝（-A）。這可以表示為：
: <math> x[n] = -A (-1)^{a[n]} \ </math>
其中，其中x[n]是[[雙極性]]比特流（-A或+ A），而a[n]是相應的二進制比特流（0或1）。

由全1組成的運行將對應於最大（正）幅值，所有0將對應於最小（負）幅值，並且交替的1和0將對應於零幅度值。 通過雙極PDM比特流的[[低通濾波器]]恢復連續振幅波形。
==例子==
[[三角函數]]，[[樣本（信號）|採樣]]的單個[[週期函數|週期]] 100倍並表示為PDM比特流，是：

0101011011110111111111111111111111011111101101101010100100100000010000000000000000000001000010010101
[[Image:Pulse-density modulation 1 period.gif|thumb|none|499px|An example of PDM of 100 samples of one period of a sine wave. 1s represented by blue, 0s represented by white, overlaid with the sine wave.]]

兩個較高頻率的正弦波週期將顯示為：

0101101111111111111101101010010000000000000100010011011101111111111111011010100100000000000000100101
[[Image:Pulse-density modulation 2 periods.gif|thumb|none|499px|A second example of PDM of 100 samples of two periods of a sine wave of twice the frequency]]

In pulse-''density'' modulation, a high ''density'' of 1s occurs at the peaks of the sine wave, while a low ''density'' of 1s occurs at the troughs of the sine wave.

== 参考资料 ==
{{Reflist}}
* [http://www.cs.tut.fi/sgn/arg/rosti/1-bit/ 1-bit A/D and D/A Converters] {{Wayback|url=http://www.cs.tut.fi/sgn/arg/rosti/1-bit/ |date=20210225050239 }}&nbsp;– Discusses [[delta modulation]], PDM (also known as Sigma-delta modulation or SDM), and relationships to [[Pulse-code modulation]] (PCM)
* {{cite web|last1=Kite|first1=Thomas|title=Understanding PDM Digital Audio|url=http://users.ece.utexas.edu/~bevans/courses/realtime/lectures/10_Data_Conversion/AP_Understanding_PDM_Digital_Audio.pdf|publisher=Audio Precision|accessdate=19 January 2017|date=2012|format=PDF|archive-date=2016-02-21|archive-url=https://web.archive.org/web/20160221175606/http://users.ece.utexas.edu/~bevans/courses/realtime/lectures/10_Data_Conversion/AP_Understanding_PDM_Digital_Audio.pdf|dead-url=no}}


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