7Byte in 8Byte packet, Lexicon LXP-1

[Tonfisch]

Hello Ctrlr's,
i need lua to convert the packed Format to Data and back.

Has someone build a funktion for that allready?
I´m on Version 5.3.201, so the bit-manipulation not so easy(for me)

[dnaldoog]

I haven't looked deeply at it because it's a head wreck all this packed bit stuff, but I think it's the same logic as with Korg and how they do it.

But I have two functions that can convert such data - you may have to change things in these functions. They return memory blocks of 8 or 7 bits depending which way you're going.

function convert728(m) -- take a set of 7 bytes - extract the msbit and save to byte 0 of 8 bytes
    assert(m:getSize() == 7)
    local b, t = {1, 2, 4, 8, 16, 32, 64}, {}
    local msb = {}
    local lsb = {}
    local mlsb = MemoryBlock()

    for i = 0, m:getSize() - 1 do -- loop each byte
        local mask = bit.rshift(bit.band(m:getByte(i), 128), 7)
        msb[i + 1] = mask * b[i + 1] -- add each msBit to table
        table.insert(lsb, bit.band(m:getByte(i), 127)) -- record the masked lsb values
    end
    local sumMsb = 0
    for i, v in ipairs(msb) do -- add each table value up for the MSB
        sumMsb = sumMsb + v
    end
    local result = MemoryBlock(sf("%.2x", bit.band(sumMsb, 127)))
    mlsb:createFromTable(lsb)
    -- now add the lsb values
    result:append(mlsb)
    -- append the lsb values to the end of the MSB
    return result
end

function convert827(m) -- take a set of 8 bytes (first byte contains) msbit data of each subsequent byte
    local t = {}
    assert(m:getSize() == 8, "size=" .. m:getSize())
    local b = {1, 2, 4, 8, 16, 32, 64}
    local msb = m:getByte(0) --
    for i = 1, m:getSize() - 1 do -- start at the second byte
        local mask = b[i]
        local band = bit.rshift(bit.band(msb, mask), i - 1)
        local addmsb = m:getByte(i) + (band * 128)
        table.insert(t, addmsb)
    end
    local result = MemoryBlock()
    result:createFromTable(t)
    assert(result:getSize() == 7)
    return result
end

[dnaldoog]

To convert an incoming data dump, find where the data starts after the sysex header (with Korg it's byte 6) less the EOX

local sizeOfLoop = m:getSize() - 1 - 6 -- m = midi:getData() from Dump
    local accrue = MemoryBlock() -- initialise holder of converted 8 bit to 7 bit
    for i = 6, sizeOfLoop, 8 do -- 6 is offset from header get the 8 bytes of data (1st byte contains MSB data)
        local set = MemoryBlock(m:getRange(i, 8)) -- get each block of 8 bytes offset by 8
        local res = convert827(set) -- convert down to 7 byte data
        accrue:append(res) -- append each 7 byte string
    end

You now have 7 bytes of data with values above 127 or containing negative values depending - Assign the data in accrue to your panel.

[Tonfisch]

Hello Dnaldoog,
looks good.
I´ll get back when i have it working.
Thank you very much!

[dnaldoog]

How did you get on @Tonfisch ?

Were these algorithms useful? 🥣

[Tonfisch]

Hello Dnaldoog,
i had problems with the memory of the LXP-1, so all i got was garbage.
Now i have it working, but i did the packet transform in my unelegant coding.
8_to_7Bit.txt
But now i can confirm that at least your "convert827" works fine.
Lesson learned: "do not program for equipment with broken Memory"
All other midi comunication is done by 4x4Bit Nibbles.
I have to fine-tune some stuff, but it is on its way.

Thank you very much!

[dnaldoog]

Your code looks pretty good to me!