# Licensed under the GPLv3 - see LICENSE
"""
Definitions for VLBI Mark 5B payloads.
Implements a Mark5BPayload class used to store payload words, and decode to
or encode from a data array.
For the specification, see
http://www.haystack.edu/tech/vlbi/mark5/docs/Mark%205B%20users%20manual.pdf
"""
from __future__ import (absolute_import, division, print_function,
unicode_literals)
import numpy as np
from collections import namedtuple
from ..vlbi_base.payload import VLBIPayloadBase
from ..vlbi_base.encoding import encode_2bit_base, decoder_levels
__all__ = ['init_luts', 'decode_2bit', 'encode_2bit',
'Mark5BPayload']
# Some duplication with mark4.py here: lut2bit = mark4.lut2bit1
# Though lut1bit = -mark4.lut1bit, so perhaps not worth combining.
[docs]def init_luts():
"""Set up the look-up tables for levels as a function of input byte.
For 1-bit mode, one has just the sign bit:
=== =====
s value
=== =====
0 -1
1 +1
=== =====
For 2-bit mode, there is a sign and a magnitude, which encode:
=== === ===== =====
m s value s*2+m
=== === ===== =====
0 0 -Hi 0
0 1 +1 2
1 0 -1 1
1 1 +Hi 3
=== === ===== =====
See Table 13 in
https://science.nrao.edu/facilities/vlba/publications/memos/upgrade/sensimemo13.pdf
and
http://www.haystack.edu/tech/vlbi/mark5/docs/Mark%205B%20users%20manual.pdf
Appendix A: sign always on even bit stream (0, 2, 4, ...), and magnitude
on adjacent odd stream (1, 3, 5, ...).
In the above table, the last column is the index in the linearly increasing
table of levels (``decoder_levels[2]``).
"""
b = np.arange(256)[:, np.newaxis]
sl = np.arange(8)
lut1bit = decoder_levels[1][((b >> sl) & 1)]
# 2-bit mode: sign bit in lower position thatn magnitude bit
# ms=00,01,10,11 = -Hi, 1, -1, Hi (lut
s = np.arange(0, 8, 2) # 0, 2, 4, 6
m = s + 1 # 1, 3, 5, 7
sl = (((b >> s) & 1) << 1) + ((b >> m) & 1)
lut2bit = decoder_levels[2][sl]
return lut1bit, lut2bit
lut1bit, lut2bit = init_luts()
# def decode_1bit(frame, nvlbichan):
# return lut1bit[frame].reshape(-1, nvlbichan)
# Decoders keyed by bits_per_sample, complex_data:
[docs]def decode_2bit(words):
b = words.view(np.uint8)
return lut2bit.take(b, axis=0)
shift2bit = np.arange(0, 8, 2).astype(np.uint8)
reorder = np.array([0, 2, 1, 3], dtype=np.uint8)
[docs]def encode_2bit(values):
bitvalues = encode_2bit_base(values.reshape(-1, 4))
# swap 1 & 2
reorder.take(bitvalues, out=bitvalues)
bitvalues <<= shift2bit
return np.bitwise_or.reduce(bitvalues, axis=-1)
encode_2bit.__doc__ = encode_2bit_base.__doc__
[docs]class Mark5BPayload(VLBIPayloadBase):
"""Container for decoding and encoding VDIF payloads.
Parameters
----------
words : `~numpy.ndarray`
Array containg LSB unsigned words (with the right size) that
encode the payload.
nchan : int, optional
Number of channels. Default: 1.
bps : int, optional
Bits per elementary sample. Default: 2.
"""
_nbytes = 2500 * 4
_encoders = {2: encode_2bit}
_decoders = {2: decode_2bit}
_sample_shape_maker = namedtuple('SampleShape', 'nchan')
def __init__(self, words, nchan=1, bps=2, complex_data=False):
if complex_data:
raise ValueError("Mark5B format does not support complex data.")
super(Mark5BPayload, self).__init__(words, sample_shape=(nchan,),
bps=bps, complex_data=False)
[docs] @classmethod
def fromdata(cls, data, bps=2):
"""Encode data as payload, using a given number of bits per sample.
It is assumed that the last dimension is the number of channels.
"""
if data.dtype.kind == 'c':
raise ValueError("Mark5B format does not support complex data.")
encoder = cls._encoders[bps]
words = encoder(data).view(cls._dtype_word)
return cls(words, nchan=data.shape[-1], bps=bps)