How to use the presto.filterbank.create_filterbank_file function in presto

raise ValueError("Bad number of channels to be written (%d). " \
                            "Check lo/hi frequencies." % new_nchans)

    print("Will extract")
    print("    %d bins (%d to %d incl.)" % (new_nspec, startbin, endbin-1))
    print("    (Original num bins: %d)" % fil.nspec)
    print("    %d channels (%d to %d incl.)" % (new_nchans, lochan, hichan-1))
    print("    (Original num chans: %d)" % fil.nchans)

    # Create output file
    outfn = options.outname % fil.header
    print("Creating out file: %s" % outfn)
    outhdr = copy.deepcopy(fil.header)
    outhdr['nchans'] = new_nchans
    outhdr['fch1'] = fil.frequencies[lochan]
    filterbank.create_filterbank_file(outfn, outhdr, nbits=fil.nbits)
    outfil = filterbank.FilterbankFile(outfn, mode='write')

    # Write data
    sys.stdout.write(" %3.0f %%\r" % 0)
    sys.stdout.flush()
    nblocks = int(new_nspec/options.block_size)
    remainder = new_nspec % options.block_size
    oldprogress = -1
    for iblock in np.arange(nblocks):
        lobin = iblock*options.block_size + startbin
        hibin = lobin+options.block_size
        spectra = fil.get_spectra(lobin, hibin)
        spectra = spectra[:,lochan:hichan] # restrict channels
        outfil.append_spectra(spectra)
        progress = int(100.0*((hibin-startbin)/new_nspec))
        if progress > oldprogress:

delays = psr_utils.delay_from_DM(dm, fil.frequencies)
        delays -= delays[np.argmax(fil.frequencies)]
        get_phases = lambda times: (times-delays)/period % 1
    else:
        get_phases = lambda times: times/period % 1

    # Create the output filterbank file
    if nbitsout is None:
        nbitsout = fil.nbits
    if inplace:
        warnings.warn("Injecting pulsar signal *in-place*")
        outfil = fil
    else:
        # Start an output file
        print("Creating out file: %s" % outfn)
        outfil = filterbank.create_filterbank_file(outfn, fil.header, \
                                            nbits=nbitsout, mode='append')

    if outfil.nbits == 8:
        raise NotImplementedError("This code is out of date. 'delays' is not " \
                                    "done in this way anymore..")
        # Read the first second of data to get the global scaling to use
        onesec = fil.get_timeslice(0, 1).copy()
        onesec_nspec = onesec.shape[0]
        times = np.atleast_2d(np.arange(onesec_nspec)*fil.tsamp).T+delays
        phases = times/period % 1
        onesec += prof(phases)
        minimum = np.min(onesec)
        median = np.median(onesec)
        # Set median to 1/3 of dynamic range
        global_scale = (256.0/3.0) / median
        del onesec

def main(fits_fn, outfn, nbits, \
            apply_weights, apply_scales, apply_offsets):
    start = time.time()
    psrfits_file = psrfits.PsrfitsFile(fits_fn)

    fil_header = translate_header(psrfits_file) 
    fil_header['nbits'] = nbits
    outfil = filterbank.create_filterbank_file(outfn, fil_header, \
                                        nbits=nbits)

    # if frequency channels are in ascending order
    # band will need to be flipped
    if psrfits_file.fits['SUBINT'].header['CHAN_BW'] > 0:
        flip_band=True
        print("\nFits file frequencies in ascending order.")
        print("\tFlipping frequency band.\n")
    else:
        flip_band=False

    # check nbits for input
    if psrfits_file.nbits < 4:
        raise ValueError('Does not support %d-bit data' % \
                        psrfits_file.nbits)

output_subints, skip):
    start = time.time()


    firstfits = pyfits.open(fits_fn[0],memmap=True)

    nchan = firstfits['SUBINT'].header['NCHAN']
    nsamps = firstfits['SUBINT'].header['NSBLK']
    nsubints = firstfits['SUBINT'].header['NAXIS2']




    fil_header = translate_header(firstfits,skip,output_subints) 
    fil_header['nbits'] = nbits
    outfil = filterbank.create_filterbank_file(outfn, fil_header, \
                                        nbits=nbits)


    # if frequency channels are in ascending order
    # band will need to be flipped
    if firstfits['SUBINT'].header['CHAN_BW'] > 0:
        flip_band=True
        print("\nFits file frequencies in ascending order.")
        print("\tFlipping frequency band.\n")
    else:
        flip_band=False

    # check nbits for input
    input_nbits = firstfits['SUBINT'].header['NBITS']
    if input_nbits < 4:
        raise ValueError('Does not support %d-bit data' % input_nbits)