New trends in DACs digital filters - Apodizing, Minimum Phase

Discussion in 'Audio Hardware' started by Averara, Jun 29, 2009.

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  1. Averara

    Averara New Member Thread Starter

    Location:
    Lisboa, Portugal
    For some time now, two manufacturers, Ayre and Meridian have been inovating in DAC filters with the Apodizing filters and lately Minimum Phase Apodizing filters.
    Wolfson has also released a DAC chip, WM8741, with 5 different filters to choose from:
    1. Linear phase 'soft knee filter'
    2. Minimum phase 'soft knee filter'
    3. Linear phase Brickwall filter
    4. Minimum phase apodizing filter
    5. linear phase apodizing filter

    Each one has its own trade-offs, but typically we can trade between phase linearity and the the presence of pre-echos.
    PS Audio has launched a digital source based on this Wolfson chip:
    http://www.psaudio.com/ps/products/detail/perfectwave-dac?cat=audio

    This blog here has a post which tries to explain the filters main characteristics in plain english, but it also contains a link to a white paper presented at the AES with a lot of detail about them:
    http://hifiduino.blogspot.com/2009/05/wm8741-digital-filters.html

    Has anyone had the chance to listen to this PS Audio PerfectWave DAC or to the Ayre CX-7e or C-5xe, or the Meridian G08.2 or 808.2 digital sources?
    If so, what were your impressions?
    Did their sound feel more 'analog', like some magazines have been publishing?
    Do you prefer it over similar priced digital sources with the typical linear phase filters?
     
  2. TONEPUB

    TONEPUB Forum Resident

    Location:
    Portland, Oregon
    We compared the 808 to the 808.2 back in our December issue. It is a definite improvement, and does sound "More analog-like" than the past model....

    The PS is just shipping this week, so at some point they will start falling into reviewers hands. I'd imagine they want to sell the ones to the paying customers first.
     
  3. Metoo

    Metoo Forum Hall Of Fame

    Location:
    Spain (EU)
    Check this out: http://en.wikipedia.org/wiki/Apodization

    From what I understand apodization in the audio context would perhaps have something to do with dithering.

    The minimum phase part would be the one to address the pre-echoes.
     
  4. Metralla

    Metralla Joined Jan 13, 2002

    Location:
    San Jose, CA
    Stereophile just got a couple of Ayre SACD players back with the MP upgrade and tested them; they really liked the improvement
     
  5. Publius

    Publius Forum Resident

    Location:
    Austin, TX
    "Apodization" is another word for "windowing".
     
  6. Alexey Lukin

    Alexey Lukin Member

    Location:
    Moscow, Russia
    I think that the term "apodizing filter" is special: it has been introduced by Peter Craven to denote the filter that "undoes" ringing of other filters in the A/D/A chain. This basically refers to the filter with sufficiently gentle frequency slope whose stopband starts lower than the transition band of other "ringing" filters.
     
    allhifi likes this.
  7. Stefan

    Stefan Senior Member

    Location:
    Montreal, Canada
    Perhaps I misunderstood the Wiki article and your statement, but this seems to suggest that these are filters that start sloping well before Nyquist, so wouldn't they basically roll off high frequency response as a compromise that's considered better sounding than ringing?
     
  8. Averara

    Averara New Member Thread Starter

    Location:
    Lisboa, Portugal
  9. Averara

    Averara New Member Thread Starter

    Location:
    Lisboa, Portugal
  10. Alexey Lukin

    Alexey Lukin Member

    Location:
    Moscow, Russia
    Wiki article refers to a different concept. If someone says "apodized filter", I'll probably think of a windowed-impulse filter. However apodizing filter is a different, special concept: it is a filter used to apodize ringing of other filters.

    Yes. Apodizing makes sense when sampling rates are high, so that there's no audible HF roll-off.
     
  11. KT88

    KT88 Senior Member

    Yes, it has some audible HF roll-off. It's a trade-off as you lose the ringing but also the transient speed and frequency extension. The linear setting is found to be best by many listeners for most well recorded discs. The new Cambridge Audio Dacmagic has three of these filter settings and sells for only $429.
    -Bill
     
  12. Averara

    Averara New Member Thread Starter

    Location:
    Lisboa, Portugal
    The Cambridge Audio Dac Magic has a Minimum Phase filter but no Apodizing:
    http://http://www.cambridgeaudio.com/summary.php?PID=320
    "Choice of filters – linear phase, minimum phase and steep filter can all be selected to suit an individual’s listening preferences"

    Also:
    http://www.cambridgeaudio.com/content.php?PID=320&COID=128&Title=Filters
    "Minimum phase meanwhile does not feature constant group delay but rather the co-efficients have been optimized without feed-forward so that the impulse response exhibits no pre-ringing in the time domain. Some commentators have argued that the pre-ringing as seen in nearly all digital filter designs may affect the transient attack of percussive instruments. Minimum phase implementation eliminates this and is a technology only seen previously in some extremely highend CD playback systems."
     
  13. KT88

    KT88 Senior Member

  14. Metoo

    Metoo Forum Hall Of Fame

    Location:
    Spain (EU)
    So, they are applying downsampling filter technology to the players. This must mean that there is some oversampling or something of the sort going on beforehand, right?

    Perhaps this, by extension, is the reason why recording at 192/24 and downsampling with a transparent algorithm delivers better transient response.
     
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