rtp implementation issues ?

Discussion in 'VOIP' started by Ram, Feb 23, 2006.

  1. Ram

    Ram Guest

    Hi,

    I am trying to implement a light weight RTP Stack on my
    own. However, it didnt work. So, i thought of studing a few
    exsisting rtp implementations.

    After observing the workings in my code for say 100000 pkts many
    times
    over and over.
    I could conclude,
    1) process scheduling
    2) Precision of select call (suppose i call select
    with 4ms delay)
    some times it would return by say 6ms or say after
    60ms.

    (u never really know).


    However, as i am going thru other implemenations. i find that
    this aspect
    is completely ignored. i really dont know whether i am wrong or not
    ?.


    However the sound quality of other implementations are better than
    my own.



    I dont know why ?. do u have any idea of how this is taken care of
    ?.


    In some code, something is said like clock slide (say this
    implementation,
    takes care of a 3% clock slide, i really dont know what this
    clock slide is ?.


    do u know what it is ?.


    In the sender, i just read from audio, attach the timestamp
    and send it in a loop.
    say, if i need 4ms data, read call does not take 4ms.it takes
    less than that
    say 1ms, sometimes 2ms and so on. ( i tested this by having
    gettimeofday before and
    after read).

    This results in a faster sender and a reciever (who picks up the
    packet from the Queue
    of packets at periodic intervals)

    i tried sleeping of the appropriate number of ms after the read
    using select and
    it didnt work ?.


    do u know how this is taken care of ?.

    In the reciever what i do is - i get a packet (say 3ms) write
    it to audio buffer and
    sleep for 3ms and then check if i have the packet and play the
    packet if it is present
    if not i play the previous packet.


    this is the logic and it sounds correct to me.


    Actually, if i say all packets are 3ms data pkts. i use select for
    1.5ms (after the
    select returns (i assume it to return before 3ms) i check for the
    pkt and do the
    appropriate thing (play last packet or play present pkt).

    (this i am doing to prevent the audio driver buffer from
    underflowing).

    please do advice, if u think i am going wrong somewhere.



    I am using 48KHZ audio, 2 channels and 4 bytes/sample.


    Regards,
    sriram
    Ram, Feb 23, 2006
    #1
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  2. Ram wrote:
    > Hi,
    >
    > I am trying to implement a light weight RTP Stack on my
    > own. However, it didnt work.


    By now you probably realized that the variance in inter-packet period
    is unavoidable.

    You can artificially get rid of this variance by matching your sampling
    rate as close as possible to your playback rate. You will need to use
    interpolation, which means you will need a buffer at the playback end,
    which means accumulation, which translates into necessary delay from
    input to output. You will need to over-sample the input data, say, at
    N-times your true input sampling rate to generate intermediate false
    samples. You can assume that the curve between two true samples is
    linear for simplicity. Create associated time stamps with the false
    samples. Then, on playback, instead of playing back real samples,
    playback the false samples corresponding to whatever the output timer
    expects. If your true input sample rate was high enough to start with
    (did not violate Nyquist Sampling Theorem), the audio should sound a
    little better after interpolation.

    -Le Chaud Lapin-
    Le Chaud Lapin, Feb 25, 2006
    #2
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