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Metamorphic's questions

Martin Hope
Metamorphic
Asked: 2018-10-25 01:52:55 +0800 CST
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根据一篇名为Pulseaudio And Latencymodule-alsa-sink的博客文章的建议，我尝试使用特殊fragments和参数加载fragment_size以确保低延迟。但是，我不确定这些到底在做什么。报告的延迟pactl list sinks与我在这些参数中输入的值没有明显的对应关系。这是我编写的用于测试各种设置的简短脚本：

pulseaudio &

# unload all sinks
pactl list sinks | grep "Owner Module" | \
    cut -d : -f 2 | \
    while read i; do
        pactl unload-module $i;
    done

# try different settings
patest () {
    pactl load-module module-alsa-sink device=$DEV \
          tsched=no \
          fixed_latency_range=yes \
          fragments=$F fragment_size=$FS > /tmp/modnum.out;
    pactl list sinks | grep Latency;
    pactl unload-module $(cat /tmp/modnum.out)
}
patest_nofix () {
    pactl load-module module-alsa-sink device=$DEV > /tmp/modnum.out;
    pactl list sinks | grep Latency;
    pactl unload-module $(cat /tmp/modnum.out)
}

fragments现在我可以尝试（F）和fragment_size（FS）的各种设置。对于硬件设备“hw:0”，延迟很低，但我看不出它与两个参数的对应关系：

F=1; FS=15; DEV=hw:0; patest
#        Latency: 2516 usec, configured 1451 usec
F=1; FS=30; DEV=hw:0; patest
#        Latency: 2484 usec, configured 1451 usec
F=5; FS=15; DEV=hw:0; patest
#        Latency: 2475 usec, configured 1451 usec
F=50; FS=200; DEV=hw:0; patest
#        Latency: 57628 usec, configured 56689 usec

# (default minimum latency:)
DEV=hw:0; patest_nofix 
#        Latency: 1983968 usec, configured 2000000 usec

对于“dmix” ALSA 设备，它有点高，不知道为什么：

F=1; FS=15; DEV=dmix; patest
#        Latency: 42752 usec, configured 42666 usec

# (default:)
DEV=dmix; patest_nofix
#        Latency: 326596 usec, configured 341333 usec

但是，当我创建自己的“dmix”设备以输出到 hw:0 时，

$ cat ~/.asoundrc
...
pcm.hw0mix {
    type dmix
    ipc_key 2498 # unique random number
    slave {
        pcm "hw:0,0"
        channels 2
        rate 48000
    }
}

然后我可以看到延迟要高得多，并且似乎固定在 0.25 秒：

F=1; FS=15; DEV=hw0mix; patest
#        Latency: 251132 usec, configured 250000 usec
F=50; FS=200; DEV=hw0mix; patest
#        Latency: 251126 usec, configured 250000 usec

# (default:)
DEV=hw0mix; patest_nofix
#        Latency: 356292 usec, configured 375000 usec

fragments参数和和“配置的延迟”之间有什么关系fragment_size，如何将配置的最小延迟更改为更低，即使对于非硬件 ALSA 设备也是如此？

pulseaudio alsa