关于AD1955工作在SACD模式下的两个问题

1. DSD模式下模拟输出有包含一个近200KHZ的高频杂信,幅度为0.2-0.3V.  但PCM模式下就正常, 很奇怪.

图1. PCM模式下, 模拟输出100Hz的正弦波

图2. DSD模式下, 模拟输出100Hz的正弦波

2. DSD模式下内部音量控制完全不起作为,始终为固定最大电平输出。

       AD1955工作在 PCM 模式时, 可以调节音量. 但是, AD1955工作在 SACD 模式下, 不能通过 SPI 写 DAC Volume Registers 调节音量, 不管我写入什么值进 Volume Registers, AD1955的模拟输出都是按Default = full volume 输出的. 

 

比如, 当前播放某一WAV文件, 此时调节音量为0, 即Volume Registers已经被写入0x0002(左声道)和0x0003(右声道), 故AD1955模拟输出为0电平信号. 此时, 切换歌曲到DSD文件, 此时模拟输出是full volume信号. 再切换到WAV, AD1955模拟输出仍为0电平信号.

 

我怀疑DSD模式下的模拟输出, 没有经过音量调节部分, 只有PCM模式下模拟输出才经过音量调节部分. 从Datasheet的AD1955内部结构图看不出, 不知道这个怀疑是否成立? 请告知, 感谢.

  • 0
    •  Analog Employees 
    on Jul 15, 2013 7:25 AM

    DSD编码的特点是在20KHZ以上动态范围会有比较大的衰减,请见下面的一些解释

    Note that high-resolution PCM (DVD-Audio, HD DVD and Blu-ray Disc) and DSD (SACD) may still technically differ at high frequencies. A reconstruction filter is typically used in PCM decoding systems, much the same way that bandwidth-limiting filters are normally used in PCM encoding systems. Any error or unwanted artifact introduced by such filters will typically affect the end-result. A claimed advantage of DSD is that product designers commonly choose to have no filtering, or modest filtering. Instead DSD leads to constant high levels of noise at these frequencies. DSD's dynamic range decreases quickly at frequencies over 20 kHz due to the use of strong noise shaping techniques which push the noise out of the audio band resulting in a rising noise floor just above 20 kHz. PCM's dynamic range, on the other hand, is the same at all frequencies. However, almost all present-day DAC chips employ some kind of sigma-delta conversion of PCM files that results in the same noise spectrum as DSD signals. (All SACD players employ an optional low-pass filter set at 50 kHz for compatibility and safety reasons, suitable for situations where amplifiers or loudspeakers cannot deliver an undistorted output if noise above 50 kHz is present in the signal.)

  • DSD下确实无法实现音量操作,不光是ADI的如此TI的也是同样,PCM码是线性码,比较容易做数据处理,DSD是信号密度编码不容易做到数据的处理,除非这个DAC的片子用的是后端的数字电位器形式,但那样的话估计模拟端的信号动态很受影响。