$ ffplay -v trace -nodisp -f lavfi "aevalsrc='mod(t,.01)*20*mod(t,-.5)+mod(t,.5)*mod(t,2)/2'"
ffplay version 3.3.5 Copyright (c) 2003-2017 the FFmpeg developers
  built with gcc 5.4.0 (Gentoo 5.4.0-r3 p1.7, pie-0.6.5)
  configuration: --prefix=/usr --libdir=/usr/lib64 --shlibdir=/usr/lib64 --docdir=/usr/share/doc/ffmpeg-3.3.5/html --mandir=/usr/share/man --enable-shared --cc=x86_64-pc-linux-gnu-gcc --cxx=x86_64-pc-linux-gnu-g++ --ar=x86_64-pc-linux-gnu-ar --optflags='-march=native -O2 -pipe' --disable-static --enable-avfilter --enable-avresample --disable-stripping --disable-indev=oss --disable-outdev=oss --enable-bzlib --disable-runtime-cpudetect --disable-debug --disable-gcrypt --disable-gnutls --disable-gmp --enable-gpl --enable-hardcoded-tables --enable-iconv --disable-lzma --enable-network --disable-openssl --enable-postproc --disable-libsmbclient --enable-ffplay --enable-sdl2 --disable-vaapi --disable-vdpau --enable-xlib --enable-libxcb --enable-libxcb-shm --enable-libxcb-xfixes --enable-zlib --disable-libcdio --disable-libiec61883 --disable-libdc1394 --disable-libcaca --disable-openal --enable-opengl --disable-libv4l2 --disable-libpulse --disable-libopencore-amrwb --disable-libopencore-amrnb --disable-libfdk-aac --disable-libopenjpeg --disable-libbluray --disable-libcelt --disable-libgme --disable-libgsm --disable-mmal --disable-libmodplug --disable-libopus --disable-libilbc --disable-librtmp --disable-libssh --disable-libschroedinger --disable-libspeex --enable-libvorbis --disable-libvpx --disable-libzvbi --disable-libbs2b --disable-chromaprint --disable-libflite --disable-frei0r --disable-libfribidi --disable-fontconfig --disable-ladspa --disable-libass --enable-libfreetype --disable-librubberband --disable-netcdf --disable-libzmq --disable-libzimg --disable-libsoxr --enable-pthreads --disable-libvo-amrwbenc --enable-libmp3lame --disable-libkvazaar --disable-nvenc --disable-libopenh264 --disable-libsnappy --disable-libtheora --disable-libtwolame --disable-libwavpack --enable-libwebp --enable-libx264 --disable-libx265 --enable-libxvid --disable-amd3dnow --disable-amd3dnowext --disable-aesni --disable-avx --disable-avx2 --disable-fma3 --disable-fma4 --disable-sse3 --disable-ssse3 --disable-sse4 --disable-sse42 --disable-xop --cpu=host --disable-doc --disable-htmlpages --enable-manpages
  libavutil      55. 58.100 / 55. 58.100
  libavcodec     57. 89.100 / 57. 89.100
  libavformat    57. 71.100 / 57. 71.100
  libavdevice    57.  6.100 / 57.  6.100
  libavfilter     6. 82.100 /  6. 82.100
  libavresample   3.  5.  0 /  3.  5.  0
  libswscale      4.  6.100 /  4.  6.100
  libswresample   2.  7.100 /  2.  7.100
  libpostproc    54.  5.100 / 54.  5.100
detected 4 logical cores  0 aq=    0KB vq=    0KB sq=    0B f=0/0
[Parsed_aevalsrc_0 @ 0x7f9308001fe0] compat: called with args=[mod(t,.01)*20*mod(t,-.5)+mod(t,.5)*mod(t,2)/2]
[Parsed_aevalsrc_0 @ 0x7f9308001fe0] Setting 'exprs' to value 'mod(t,.01)*20*mod(t,-.5)+mod(t,.5)*mod(t,2)/2'
[out @ 0x7f9308003670] auto-inserting filter 'auto_resampler_0' between the filter 'Parsed_aevalsrc_0' and the filter 'out'
[AVFilterGraph @ 0x7f93080016f0] query_formats: 2 queried, 0 merged, 3 already done, 0 delayed
[auto_resampler_0 @ 0x7f9308003cf0] picking dbl out of 5 ref:dblp
[Parsed_aevalsrc_0 @ 0x7f9308001fe0] sample_rate:44100 chlayout:mono duration:-1
[auto_resampler_0 @ 0x7f9308003cf0] [SWR @ 0x7f9308003e60] Using dblp internally between filters
[auto_resampler_0 @ 0x7f9308003cf0] ch:1 chl:mono fmt:dblp r:44100Hz -> ch:1 chl:mono fmt:dbl r:44100Hz
[lavfi @ 0x7f93080008f0] All info found
[lavfi @ 0x7f93080008f0] stream 0: start_time: 0.000 duration: -209146758205323.719
[lavfi @ 0x7f93080008f0] format: start_time: 0.000 duration: -9223372036854.775 bitrate=2822 kb/s
Input #0, lavfi, from 'aevalsrc='mod(t,.01)*20*mod(t,-.5)+mod(t,.5)*mod(t,2)/2'':
  Duration: N/A, start: 0.000000, bitrate: 2822 kb/s
    Stream #0:0, 1, 1/44100: Audio: pcm_f64le, 44100 Hz, mono, dbl, 2822 kb/s
[ffplay_abuffer @ 0x7f93080202a0] Setting 'sample_rate' to value '44100'
[ffplay_abuffer @ 0x7f93080202a0] Setting 'sample_fmt' to value 'dbl'
[ffplay_abuffer @ 0x7f93080202a0] Setting 'channels' to value '1'
[ffplay_abuffer @ 0x7f93080202a0] Setting 'time_base' to value '1/44100'
[ffplay_abuffer @ 0x7f93080202a0] Setting 'channel_layout' to value '0x4'
[ffplay_abuffer @ 0x7f93080202a0] tb:1/44100 samplefmt:dbl samplerate:44100 chlayout:0x4
[ffplay_abuffersink @ 0x7f9308020760] auto-inserting filter 'auto_resampler_0' between the filter 'ffplay_abuffer' and the filter 'ffplay_abuffersink'
[AVFilterGraph @ 0x7f9308003990] query_formats: 2 queried, 0 merged, 3 already done, 0 delayed
[auto_resampler_0 @ 0x7f9308020bc0] [SWR @ 0x7f9308020d10] Using dblp internally between filters
[auto_resampler_0 @ 0x7f9308020bc0] ch:1 chl:mono fmt:dbl r:44100Hz -> ch:1 chl:mono fmt:s16 r:44100Hz
Audio frame changed from rate:44100 ch:1 fmt:dbl layout:mono serial:-1 to rate:44100 ch:1 fmt:dbl layout:mono serial:1
[ffplay_abuffer @ 0x7f92e8002ee0] Setting 'sample_rate' to value '44100'
[ffplay_abuffer @ 0x7f92e8002ee0] Setting 'sample_fmt' to value 'dbl'
[ffplay_abuffer @ 0x7f92e8002ee0] Setting 'channels' to value '1'
[ffplay_abuffer @ 0x7f92e8002ee0] Setting 'time_base' to value '1/44100'
[ffplay_abuffer @ 0x7f92e8002ee0] Setting 'channel_layout' to value '0x4'
[ffplay_abuffer @ 0x7f92e8002ee0] tb:1/44100 samplefmt:dbl samplerate:44100 chlayout:0x4
[ffplay_abuffersink @ 0x7f92e80035b0] auto-inserting filter 'auto_resampler_0' between the filter 'ffplay_abuffer' and the filter 'ffplay_abuffersink'
[AVFilterGraph @ 0x7f92e8002bb0] query_formats: 2 queried, 0 merged, 3 already done, 0 delayed
[auto_resampler_0 @ 0x7f92e8003a80] [SWR @ 0x7f92e8003bd0] Using dblp internally between filters
[auto_resampler_0 @ 0x7f92e8003a80] ch:1 chl:mono fmt:dbl r:44100Hz -> ch:1 chl:mono fmt:s16 r:44100Hz
    nan M-A:    nan fd=   0 aq=  356KB vq=    0KB sq=    0B f=0/0